Method and device for transmitting data

ABSTRACT

A method and a device for transmitting data in in a wireless local area network are provided. The device supports a multiple basic service set and sends a physical protocol data unit (PPDU). The PPDU includes an identifier used to assist the device in identifying a basic service set.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a continuation application of a currentlypending U.S. application Ser. No. 15/390,607 having a U.S. filing dateof Dec. 26, 2016. The U.S. application Ser. No. 15/390,607 is a BypassContinuation Application of an international application No.PCT/IB2015/001797 having an international filing date of 26 Jun. 2015and designating the United States, the international applicationclaiming priority to the following earlier filed Korean patentapplications

-   -   No. 10-2014-0080169 filed on Jun. 27, 2014,    -   No. 10-2014-0080170 filed on Jun. 27, 2014,    -   No. 10-2014-0080171 filed on Jun. 27, 2014,    -   No. 10-2014-0080172 filed on Jun. 27, 2014,    -   No. 10-2014-0080173 filed on Jun. 27, 2014, and    -   No. 10-2014-0080174 filed on Jun. 27, 2014.

The entire contents of the aforesaid U.S. application, the internationalapplication, and the afore-listed Korean patent applications areincorporated herein by reference. The applicant claims the benefit ofand claims priory herein to all of these applications and their filingdates and priority dates.

BACKGROUND OF THE INVENTION Field of the invention

The present invention relates to wireless communications, and moreparticularly, to a method and device for transmitting data in a wirelesslocal area network.

Related Art

Institute of Electrical and Electronics Engineers (IEEE) 802.11nstandard established in 2009 provides a transfer rate of up to 600 Mbpsat a frequency band of 2.4 GHz or 5 GHz on the basis of Multiple InputMultiple Output (MIMO) technique.

IEEE 802.11ac standard established in 2013 aims to provide a throughputgreater than or equal to 1 Gbps utilizing Medium Access Control (MAC)Service Access Point (SAP) layer scheme at a frequency band less than orequal to 6 GHz. A system supporting IEEE 802.11ac standard is referredto as a Very High Throughput (VHT) system.

There are continuing efforts to implement more effective Wireless LocalArea Network (WLAN) technologies in increasingly congested environments.

SUMMARY OF THE INVENTION

The present invention provides a method for transmitting data in awireless local area network.

The present invention provides a device for transmitting data in awireless local area network.

The present invention provides a method for supporting multiple basicservice set in a wireless local area network.

The present invention also provides a device for supporting multiplebasic service set in a wireless local area network.

In an aspect, a method for supporting multiple basic service set in awireless local area network is provided. The method includes receiving,by a transmitting station, a first frame from an access point (AP), thefirst frame including a multiple basic service set identifier (BSSID)information indicating a plurality of BSSIDs to assign a plurality ofBSSs, each of the plurality of BSSIDs having 48 bits uniquelyidentifying a corresponding BSS, receiving, by the transmitting station,a second frame from the AP, the second frame including a firstidentifier, the first identifier having 3 bits used to assist thetransmitting station in identifying at least one of the plurality ofBSSs, and transmitting, by the transmitting station, a physical protocoldata unit (PPDU) to a receiving station, the PPDU including a signalfield, the signal field including a second identifier, the secondidentifier being set to a value of the first identifier. The value ofthe first identifier is set to a same value for all of the plurality ofBSSs.

The signal field may further include an indication field having one bitindicating that the second identifier is present.

In another aspect, a device configured for supporting multiple basicservice set in a wireless local area network is provided. The deviceincludes a radio frequency module configured to transmit and receiveradio signals and a processor operatively coupled with the radiofrequency module and configured to instruct the radio frequency moduleto receive a first frame from an access point (AP), the first frameincluding a multiple basic service set identifier (BSSID) informationindicating a plurality of BSSIDs to assign a plurality of BSSs, each ofthe plurality of BSSIDs having 48 bits uniquely identifying acorresponding BSS, instruct the radio frequency module to receive asecond frame from the AP, the second frame including a first identifier,the first identifier having 3 bits used to assist the device inidentifying at least one of the plurality of BSSs, and instruct theradio frequency module to transmit a physical protocol data unit (PPDU)to a receiving station, the PPDU including a signal field, the signalfield including a second identifier, the second identifier being set toa value of the first identifier. The value of the first identifier isset to a same value for all of the plurality of BSSs.

An access point can manage multiple virtual access points.

Since a greater amount of data can be transmitted during a same timeperiod, a transmission efficiency can be increased. In addition, aPeak-to-Average Power Ratio (PAPR) of a transmitter can be decreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows PPDU formats used by the legacy system.

FIG. 2 shows an HEW PPDU format according to an embodiment of thepresent invention.

FIG. 3 shows constellation phases for the conventional PPDU.

FIG. 4 shows constellation phases for a proposed HEW PPDU.

FIG. 5 shows an HEW PPDU format in a 20 MHz channel.

FIG. 6 shows an HEW PPDU format in a 40 MHz channel.

FIG. 7 shows an HEW PPDU format in an 80 MHz channel.

FIG. 8 shows a PPDU format according to another embodiment of thepresent invention.

FIG. 9 shows bandwidth signaling according to an embodiment of thepresent invention.

FIG. 10 shows a Direct Sequence Spread Spectrum (DSSS) PPDU format usedin IEEE 802.11b/g.

FIG. 11 shows data transmission according to an embodiment of thepresent invention.

FIG. 12 shows data transmission according to another embodiment of thepresent invention.

FIG. 13 shows an HEW PPDU format according to an embodiment of thepresent invention.

FIG. 14 shows a Multiple BSSID element format according to an embodimentof the present invention.

FIG. 15 shows a Multiple BSSID index element format according to anembodiment of the present invention.

FIG. 16 shows an example of PPDU transmission having an RTS/CTSbandwidth signal.

FIG. 17 shows a scrambling procedure for a data field in a PPDU.

FIG. 18 shows an example of HEW PPDU transmission having an RTS/CTSbandwidth signal.

FIG. 19 shows a PIFS Recovery procedure performed after a frame erroroccurs in the middle of TXOP.

FIG. 20 shows a Recovery procedure when a frame error occurs.

FIG. 21 shows an example of a Medium Access Control (MAC) frame formatbased on the conventional IEEE 802.11.

FIG. 22 shows another example of a MAC frame format based on theconventional IEEE 802.11.

FIG. 23 shows an A-MPDU format according to an aggregation scheme for anMPDU.

FIG. 24 shows a frame format according to an embodiment of the presentinvention.

FIG. 25 shows an A-MPDU format having a PV0 Null Data frame.

FIG. 26 is a block diagram of an STA according to an embodiment of thepresent invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The proposed wireless local area network (WLAN) system may operate at aband less than or equal to 6 GHz or at a band of 60 GHz. The operatingband less than or equal to 6 GHz may include at least one of 2.4 GHz and5 GHz.

For clarity, a system complying with the Institute of Electrical andElectronics Engineers (IEEE) 802.11 a/g standard is referred to as anon-High Throughput (non-HT) system, a system complying with the IEEE802.11n standard is referred to as a High Throughput (HT) system, and asystem complying with IEEE 802.11ac standard is referred to as a VeryHigh Throughput (VHT) system. In comparison thereto, a WLAN systemcomplying with the proposed method is referred to as a High EfficiencyWLAN (HEW) system. A WLAN system supporting systems used before the HEWsystem is released is referred to as a legacy system. The HEW system mayinclude an HEW Station (STA) and an HEW Access Point (AP). The term HEWis only for the purpose of distinguishing from the conventional WLAN,and there is no restriction thereon. The HEW system may support IEEE802.11/a/g/n/ac by providing backward compatibility in addition to theproposed method.

Hereinafter, unless a function of a station (STA) is additionallydistinguished from a function of an Access Point (AP), the STA mayinclude a non-AP STA and/or the AP. When it is described as an STA-to-APcommunication, the STA may be expressed as the non-AP STA, and maycorrespond to communication between the non-AP STA and the AP. When itis described as STA-to-STA communication or when a function of the AP isnot additionally required, the STA may be the non-AP STA or the AP.

A Physical layer Protocol Data unit (PPDU) is a data unit for datatransmission.

FIG. 1 shows PPDU formats used by the legacy system.

A non-HT PPDU supporting IEEE 802.11a/g includes a Legacy-Short TrainingField (L-STF), a Legacy-Long Training Field (L-LTF), and a Legacy-Signal(L-SIG).

An HT PPDU supporting IEEE 802.11n includes a HT-SIG, a HT-STF, and aHT-LTF after the L-SIG.

A VHT PPDU supporting IEEE 802.11ac includes a VHT-SIG-A, a VHT-STF, aVHT-LTF, and a VHT-SIG-B after the L-SIG.

FIG. 2 shows an HEW PPDU format according to an embodiment of thepresent invention.

An L-STF may be used for frame detection, Automatic Gain Control (AGC),diversity detection, and coarse frequency/time synchronization.

An L-LTF may be used for fine frequency/time synchronization and channelestimation.

An L-SIG may include information indicating a total length of acorresponding PPDU (or information indicating a transmission time of aphysical layer protocol service unit (PSDU)).

The L-STF, the L-LTF and the L-SIG may be identical to L-STF, L-LTF andL-SIG of the VHT system. The L-STF, the L-LTF and the L-SIG may bereferred to as a legacy portion. The L-STF, the L-LTF, and the L-SIG maybe transmitted in at least one Orthogonal Frequency DivisionMultiplexing (OFDM) symbol generated on the basis of 64-points FastFourier Transform (FFT) (or 64 subcarriers) in each 20 MHz channel. For20 MHz transmission, the legacy portion may be generated by performingan inverse Discrete Fourier Transform (IDFT) with 64 FFT points. For 40MHz transmission, the legacy portion may be generated by performing anIDFT with 128 FFT points. For 80 MHz transmission, the legacy portionmay be generated by performing an IDFT with 512 FFT points.

A HEW-SIGA may include common control information commonly received byan STA which receives a PPDU. The HEW-SIGA may be transmitted in 2 OFDMsymbols or 3 OFDM symbols.

The following table exemplifies information included in the HEW-SIGA. Afield name or the number of bits is for exemplary purposes only.

TABLE 1 Field Bits Description Bandwidth  2 Set to 0 for 20 MHz, 1 for40 MHz, 2 for 80 MHz, 3 for 160 MHz and 80 + 80 MHz mode STBC  1 Set to1 if all streams use STBC, otherwise set to 0. When STBC bit is 1, anodd number of space time streams per user is not allowed. Group ID  6Set to the value of the TXVECTOR parameter GROUP_ID. A value of 0 or 63indicates a HEW SU PPDU; otherwise, indicates a HEW MU PPDU.Nsts/Partial AID 12 For MU: 3 bits/user with maximum of 4 users Set to 0for 0 space time streams Set to 1 for 1 space time stream Set to 2 for 2space time streams Set to 3 for 3 space time streams Set to 4 for 4space time streams Otherwise: first 3 bits contain stream allocation forSU, set to 0 for 1 space time stream, set to 1 for 2 space time streams,etcetera up to 8 streams. Remaining 9 bits contain partial associationidentifier (AID). No TXOP PS  1 Set to 1 to indicate that TXOP PS is notallowed. Set to 0 to indicate that TXOP PS is allowed. Set to the samevalue in all PPDUs in downlink MU TXOP. GI (Guard  2 Set B0 to 0 forLong GI, set to 1 for Short GI. Set B1 to 1 when interval) Short GI.Coding  2 For SU: Set B2 to 0 for BCC, set to 1 for LDPC For MU: Set B2to 0 for BCC, set to 1 for LDPC for 1st user If user 1 has 0 Nsts value,then B2 is reserved and set to 1 MCS  4 For SU/Broadcast/Multicast:Modulation and coding scheme (MCS) index For MU: B1: Set to 0 for BCC, 1for LDPC for the 2nd user B2: Set to 0 for BCC, 1 for LDPC for the 3rduser B3: Set to 0 for BCC, 1 for LDPC for the 4th user If user 2, 3, or4 has 0 Nsts value, then corresponding bit is reserved and set to 1 SU- 1 Set to 1 when packet is a SU-beamformed packet Beamformed Set to 0otherwise For MU: Reserved, set to 1 CRC  8 Tail  6 All zeros

A HEW-STF may be used to improve an AGC estimation in an MIMOtransmission.

A HEW-LTF may be used to estimate a MIMO channel. The HEW-LTF may startat the same point of time and may end at the same point of time acrossall users.

A HEW-SIGB may include user-specific information required for each STAto receive its PSDU. For example, the HEW-SIGB may include informationregarding a length of a corresponding PSDU and/or a bandwidth or channelin which the PSDU for a corresponding receiver is transmitted.

A data portion may include at least one PSDU. The position of theHEW-SIGB is illustration purpose only. The HEW-SIGB may be followed bythe data portion. The HEW-SIGB may be followed by the HEW-STF or theHEW-LTF.

In the proposed PPDU format, the number of OFDM subcarriers may beincreased per unit frequency. The number of OFDM subcarriers mayincrease K-times by increasing FFT size. K may be 2, 4, or 8. Thisincrease may be accomplished via downclocking (e,g, using a larger FFTsize with a same sampling rate).

For example, K=4 downclocking is assumed. As for the legacy portion, 64FFT is used in a 20 MHz channel, 128 FFT is used in a 40 MHz channel,and 256 FFT is used in an 80 MHz channel. As for a HEW portion using thelarger FFT size, 256 FFT is used in a 20 MHz channel, 512 FFT is used ina 40 MHz channel, and 1024 FFT is used in an 80 MHz channel. TheHEW-SIGA may have same FFT size as the legacy portion. The HEW portionmay have larger FFT size than the legacy portion.

The PPDU is generated by performing IDFT with two different FFT sizes.The PPDU may include a first part with a first FFT size and a secondpart with a second FFT size. The first part may include at least one ofthe L-STF, the L-LTF, the L-SIG and the HEW-SIGA. The second part mayinclude at least one of the HEW-STF, the HEW-LTF and the data portion.The HEW-SIGB may be included in the first part or in the second part.

When an FFT size is increased, an OFDM subcarrier spacing is decreasedand thus the number of OFDM subcarriers per unit frequency is increased,but an OFDM symbol duration is increased. A guard interval (GI) (or alsoreferred to as a Cyclic Prefix (CP) length) of the OFDM symbol time canbe decreased when the FFT size is increased.

If the number of OFDM subcarriers per unit frequency is increased, alegacy STA supporting the conventional IEEE 80.2.11a/g/n/ac cannotdecode a corresponding PPDU. In order for the legacy STA and an HEW STAto co-exist, L-STF, L-LTF, and L-SIG are transmitted through 64 FFT in a20 MHz channel so that the legacy STA can receive the L-STF, the L-LTF,and the L-SIG. For example, the L-SIG is transmitted in a single OFDMsymbol, a symbol time of the single OFDM symbol is 4 micro seconds (us),and the GI is 0.8 us.

Although the HEW-SIGA includes information required to decode an HEWPPDU by the HEW STA, the HEW-SIGA may be transmitted through 64 FFT inan 20 MHz channel so that it can be received by both of the legacy STAand the HEW STA. This is to allow the HEW STA to receive not only theHEW PPDU but also the conventional non-HT/HT/VHT PPDU.

FIG. 3 shows constellation phases for the conventional PPDU.

To identify a format of a PPDU, a phase of a constellation for two OFDMsymbols transmitted after L-STF, L-LTF, and L-SIG is used.

A ‘first OFDM symbol’ is an OFDM symbol first appeared after the L-SIG.A ‘second OFDM symbol’ is an OFDM symbol subsequent to the first OFDMsymbol.

In a non-HT PPDU, the same phase of the constellation is used in the 1stOFDM symbol and the 2nd OFDM symbol. Binary Phase Shift Keying (BPSK) isused in both of the 1st OFMD symbol and the 2nd OFDM symbol.

In an HT PPDU, although the same phase of the constellation is used inthe 1st OFDM symbol and the 2nd OFDM symbol, the constellation rotatesby 90 degrees in a counterclockwise direction with respect to the phaseused in the non-HT PPDU. A modulation scheme having a constellationwhich rotates by 90 degrees is called Quadrature Binary Phase ShiftKeying (QBPSK).

In a VHT PPDU, a constellation of the first OFDM symbol is identical tothat of the non-HT PPDU, whereas a constellation of the second OFDMsymbol is identical to that of the HT PPDU. The constellation of secondOFDM symbol rotates 90 degrees in a counterclockwise direction withrespect to the 1st OFDM symbol. The first OFDM symbol uses BPSKmodulation, and the 2nd OFDM symbol uses QBPSK modulation. SinceVHT-SIG-A is transmitted after L-SIG and the VHT-SIG-A is transmitted intwo OFDM symbols, the first OFDM symbol and the second OFDM symbol areused to transmit the VHT-SIG-A.

FIG. 4 shows constellation phases for a proposed HEW PPDU.

To distinguish from a non-HT/HT/VHT PPDU, a constellation of at leastone OFDM symbol transmitted after L-SIG can be used.

Just like the non-HT PPDU, a first OFDM symbol and a second OFDM symbolof the HEW PPDU have the same constellation phase. A BPSK modulation maybe used for the first OFDM symbol and the second OFDM symbol. The STAcan differentiate the HEW PPDU and HT/VHT PPDUs.

In an embodiment, to differentiate the HEW PPDU and the non-HT PPDU, theconstellation of a third OFDM symbol can be utilized. The constellationof the third OFDM symbol may rotate by 90 degrees in a counterclockwisedirection with respect to the second OFDM symbol. The first and secondOFDM symbols may use BPSK modulation, but the third OFDM symbol may useQBPSK modulation.

In another embodiment, the HEW-SIGA may provide an indication about theformat of the PPDU. The indication may indicate whether the format ofthe PPDU is a HEW PPDU. The HEW-SIGA may provide an indication about ause of orthogonal frequency division multiple access (OFDMA).

Hereinafter, a PPDU using a phase rotation in frequency domain isproposed in order to support lower Peak-to-Average Power Ratio (PAPR).

For clarity, it is assumed that the second part (i.e. HEW part) of thePPDU uses 4-times FFT size via downclocking.

Hereinafter, a subchannel refers to a resource allocation unit to beallocated to a STA. Operating bandwidth (i.e. 20 MHz channel, 40 MHzchannel, 80 MHz channel or 160 MHz channel) can be divided into aplurality of subchannels. A subchannel may include one or moresubcarriers. The plurality of subchannels may have same number ofsubcarriers or different number of subcarriers. One or more subchannelscan be allocated to the STA. The STA can transmit one or more PPDUsthrough the allocated subchannels. The subchannel may be referred to as‘a subband’ or ‘a subgroup’.

FIG. 5 shows an HEW PPDU format in a 20 MHz channel.

The first part (i.e. L-LTF, L-LTF, L-SIG and HEW-SIGA) uses 64 FFT inthe 20 MHz channel. In order to implement the 256 FFT in the secondpart, it is proposed to decrease an overhead by performing ¼down-clocking on a VHT 80 MHz PPDU format and by decreasing GI to 0.8 usand 0.4 us.

If the VHT 80 MHz PPDU format is subjected to ¼ down-clocking, an OFDMsymbol time is increased by four times, and thus is 16 us when usingLong GI, and is 14.4 us when using Short GI. That is, the GI is alsoincreased to 3.2 us in case of Long GI and to 1.6 us in case of ShortGI. However, the GI may keep to 0.8 us in case of Long GI and to 0.4 usin case of Short GI. In doing so, after performing the ¼ downclocking,the OFDM symbol time is 13.6 us when using Long GI and is 13.2 us whenusing Short GI.

If the VHT 80 MHz PPDU format is subjected to ¼ down-clocking in the 20MHz channel, each of 64 FFT-based VHT-STF, VHT-LTF, and VHT-SIG-B mayconstitute one subchannel, and as a result, 4 subchannels are combinedand transmitted through the 20 MHz channel in unit of 256 FFT.

In FIG. 5, in order to decrease a Peak-to-Average Power Ratio (PAPR) ofa transmitter STA, the second part may be subjected to multiplicationfor a phase waveform in unit of subchannel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},{k \geq {- 64}}} \\{{+ 1},{k < {- 64}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Herein, R(k) denotes a multiplication value for a phase waveform at asubcarrier index k. 256 subcarriers are divided into 4 subchannels.Respective subchannel is composed of 64 subcarriers. A sequence {+1, −1,−1, −1} may be multiplied for the 4 subchannels, starting from asubchannel having a smallest subcarrier index, that is, a lowermostsubchannel. The number of subchannels and the sequence {+1, −1, −1, −1}are exemplary purpose only. 256 subcarriers may be divided into aplurality of subchannels and respective subchannel may be phase-rotatedby multiplying +1 or −1.

The equation 1 can be expressed as follows. The 256 subcarriers aredivided into first and second subgroups that have different number ofsubcarriers. The first subgroup is phase-rotated by multiplying +1 andthe second subgroup is phase-rotated by multiplying −1.

A sequence constituting the HEW-STF and the HEW-LTF may be as follows.HEW-STF={HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58)},HEW-LTF={LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1 , −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft , 1,LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright}

where:

HTS_(−58,58)=√{square root over (½)} {0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0,1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0,−1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0,1+j, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0,0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0,0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0,0, 1+j, 0, 0},

LTFleft={1, 1, −1, −1, 1, 1, −1, 1, −1, 1, 1, 1, 1, 1, 1, −1, −1, 1, 1,−1, 1, −1, 1, 1, 1, 1, 1},

LTFright={1, −1, −1, 1, 1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, −1,−1, 1, −1, 1, −1, 1, 1, 1, 1}.

FIG. 6 shows an HEW PPDU format in a 40 MHz channel.

In order to implement the 512 FFT in the 40 MHz channel, it is proposedto use two blocks for the aforementioned 256 FFT transmission of the 20MHz channel. Like in the 256 FFT transmission in the 20 MHz channel, anOFDM symbol time is 13.6 us when using Long GI, and is 13.2 us whenusing Short GI.

L-STF, L-LTF, L-SIG, and HEW-SIGA are generated using 64 FFT and aretransmitted in a duplicated manner two times in the 40 MHz channel. Thatis, the first part is transmitted in a first 20 MHz subchannel and itsduplication is transmitted in a second 20 MHz subchannel.

In order to decrease a PAPR of a transmitter STA for transmitting theL-STF, the L-LTF, the L-SIG, and the HEW-SIGA, multiplication may beperformed on a phase waveform in unit of 20 MHz channel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{+ j},{k \geq 0}} \\{{+ 1},{k < 0}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 2} \right\rbrack\end{matrix}$

This means that the first part is phase-rotated by multiplying +1 forthe first 20 MHz subchannel and is phase-rotated by multiplying +j forthe second 20 MHz subchannel.

The equation 2 can be expressed as follows. The 128 subcarriers aredivided into first and second subgroups. The first subgroup isphase-rotated by multiplying +1 and the second subgroup is phase-rotatedby multiplying +j.

For each 64 FFT-based subchannel constituting 512 FFT, in order todecrease a PAPR of a transmitter STA for transmitting HEW-STF, HEW-LTF,and HEW-SIGB, multiplication may be performed on a phase waveform inunit of subchannel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},{64 \leq k}} \\{{+ 1},{0 \leq k < 64}} \\{{- 1},{{- 192} \leq k < 0}} \\{{+ 1},{k < {- 192}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 3} \right\rbrack\end{matrix}$

More specifically, according to Equation 3, 512 subcarriers are dividedinto 8 subchannels. Respective subchannel is composed of 64 subcarriers.A sequence {+1, −1, −1, −1, +1, −1, −1, −1} may be multiplied for the 8subchannels, starting from a subchannel having a smallest subcarrierindex, that is, a lowermost subchannel.

The equation 3 can be expressed as follows. The 512 subcarriers aredivided into four subgroups. The first subgroup is phase-rotated bymultiplying +1, the second subgroup is phase-rotated by multiplying −1,the third subgroup is phase-rotated by multiplying +1, and the fourthsubgroup is phase-rotated by multiplying −1.

A sequence constituting the HEW-STF and the HEW-LTF may be as follows.

HEW-STF ={HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58),0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0,0, 0, 0, HTS_(−58,58)},

HEW-LTF={LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft , 1,LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, LTFleft , 1, LTFright , −1, −1, −1, 1,1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright, 1 , −1, 1, −1, 0, 0, 0, 1,−1, −1, 1, LTFleft , 1, LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1,−1, LTFleft, 1, LTFright}

Herein,

HTS_(−58,58)=√{square root over (½)}

{0,0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0,0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0,0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j,0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0,0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0},

LTFleft={1, 1, −1, −1, 1, 1, −1, 1, −1, 1, 1, 1, 1, 1, 1, −1, −1, 1, 1,−1, 1, −1, 1, 1, 1, 1},

LTFright={1, −1, −1, 1, 1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, −1,−1, 1, −1, 1, −1, 1, 1, 1, 1}.

FIG. 7 shows an HEW PPDU format in an 80 MHz channel.

In order to implement the 1024 FFT in the 80 MHz channel, it is proposedto use four blocks for the aforementioned 256 FFT transmission of the 20MHz channel. Like in the 256 FFT transmission in the 20 MHz channel, anOFDM symbol time is 13.6 us when using Long GI, and is 13.2 us whenusing Short GI.

L-STF, L-LTF, L-SIG, and HEW-SIGA which are transmitted using 64 FFT arealso transmitted in a duplicated manner four times in the 80 MHzchannel. That is, the first part is transmitted in a first 20 MHzsubchannel and its duplications are transmitted in second, third andfourth 20 MHz subchannels respectively.

In order to decrease a PAPR of a transmitter STA for transmitting theL-STF, the L-LTF, the L-SIG, and the HEW-SIGA, multiplication may beperformed on a phase waveform in unit of 20 MHz channel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{- 1},{k \geq {- 64}}} \\{{+ 1},{k < {- 64}}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 4} \right\rbrack\end{matrix}$

This means that the first part is phase-rotated by multiplying +1 forthe first 20 MHz subchannel and is phase-rotated by multiplying −1 forthe second, third and fourth 20 MHz subchannels.

The equation 4 can be expressed as follows. The 256 subcarriers aredivided into first and second subgroups that have different number ofsubcarriers. The first subgroup is phase-rotated by multiplying +1 andthe second subgroup is phase-rotated by multiplying −1.

For each 64 FFT-based subchannel constituting 1024 FFT, in order todecrease a PAPR of a transmitter STA for transmitting HEW-STF, HEW-LTF,and HEW-SIGB, multiplication may be performed on a phase waveform inunit of subchannel as follows.

$\begin{matrix}{{R(k)} = \left\{ \begin{matrix}{{{- 1},256} \leq k} \\{{{+ 1},192} \leq k < 256} \\{{{- 1},64} \leq k < 192} \\{{{+ 1},0} \leq {k64}} \\{{- 1},{{- 192} \leq k < 0}} \\{{{+ 1},256} \leq k \leq {- 192}} \\{{- 1},{{- 448} \leq k < {- 256}}} \\{{{+ 1},k} < {- 448}}\end{matrix} \right.} & \left\lbrack {{Equation}\mspace{14mu} 5} \right\rbrack\end{matrix}$

More specifically, according to Equation 5, 1024 subcarriers are dividedinto 16 subchannels. Respective subchannel is composed of 64subcarriers. A sequence {+1, −1, −1, −1, +1, −1, −1, −1, +1, −1, −1, −1,+1, −1, −1, −1} may be multiplied for the 16 subchannels, starting froma subchannel having a smallest subcarrier index, that is, a lowermostsubchannel.

The equation 5 can be expressed as follows. The 1024 subcarriers aredivided into 8 subgroups. The first subgroup is phase-rotated bymultiplying +1, the second subgroup is phase-rotated by multiplying −1,the third subgroup is phase-rotated by multiplying +1, the fourthsubgroup is phase-rotated by multiplying −1, the fifth subgroup isphase-rotated by multiplying +1, the sixth subgroup is phase-rotated bymultiplying −1, the seventh subgroup is phase-rotated by multiplying +1and the eighth subgroup is phase-rotated by multiplying −1.

A sequence constituting the HEW-STF and the HEW-LTF is as follows.

HEW-STF={HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0,0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0, 0,0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,0, HTS_(−58,58), 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, HTS_(−58,58)},

HEW-LTF={LTFleft, 1, LTFright, −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1,LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft , 1,LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright,0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, LTFleft , 1, LTFright , −1, −1, −1, 1,1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright, 1 , −1, 1, −1, 0, 0, 0, 1,−1, −1, 1, LTFleft , 1, LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1,−1, LTFleft, 1, LTFright, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, LTFleft , 1,LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright,1 , −1, 1, −1, 0, 0, 0, 1, −1, −1, 1, LTFleft , 1, LTFright , −1, −1,−1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft, 1, LTFright, 0, 0, 0, 0, 0, 0,0, 0, 0, 0, 0, LTFleft , 1, LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1,1, −1, LTFleft, 1, LTFright, 1, −1, 1, −1, 0, 0, 0, 1, −1, −1, 1,LTFleft , 1, LTFright , −1, −1, −1, 1, 1, −1, 1, −1, 1, 1, −1, LTFleft,1, LTFright},

Herein,

HTS_(−58,58)=√{square root over (½)}{ 0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0,1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0, 0,−1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0,1+j, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1+j, 0, 0, 0, −1−j, 0,0, 0, 1+j, 0, 0, 0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 0, 0, 0,0, −1−j, 0, 0, 0, −1−j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0, 0, 1+j, 0, 0,0, 1+j, 0, 0},

LTFleft={1, 1, 1, −1, −1, 1, 1, −1, 1, −1, 1, 1, 1, 1, 1, 1, −1, −1, 1,1, −1, 1, −1, 1, 1, 1, 1},

LTFright={1, −1, −1, 1, 1, −1, 1, −1, 1, −1, −1, −1, −1, −1, 1, 1, −1,−1, 1, −1, 1, −1, 1, 1, 1, 1}

An FFT size can be increased to improve PPDU transmission efficiency. Inorder to provide compatibility with the legacy STA, the first part (STF,LTF, L-SIG and HEW-SIGA) using the same FFT size as the legacy PPDU isfirst transmitted, and subsequently the second part (HEW-STF, HEW-LTF,HEW-SIGB and a PSDU) using a larger FFT size are transmitted.

In order to decrease a PAPR of a transmitter STA, the first part and thesecond part uses different phase rotation in frequency domain. It meansthat a phase rotation for subcarriers in the first part is differentfrom a phase rotation for subcarriers in the second part.

FIG. 8 shows a PPDU format according to another embodiment of thepresent invention.

Since the number of OFDM subcarriers per unit frequency increases aftertransmitting L-STF, L-LTF, L-SIG, and HEW-SIGA, a processing time may berequired to process data with larger FFT size. The processing time maybe called an HEW transition gap.

In an embodiment, the HEW transition gap may be implemented by defininga Short Inter-Frame Space (SIFS) followed by the HEW-STF. The SIFS maybe positioned between the HEW-SIGA and the HEW-STF. The SIFS may bepositioned between the HEW-SIGB and the HEW-STF.

In another embodiment, the HEW transition gap may be implemented in sucha manner that the HEW-STF is transmitted one more time. The duration ofthe HEW-STF may vary depending on the processing time or STA'scapability. If the processing time is required, the duration of theHEW-STF may become double.

Hereinafter, a proposed bandwidth signaling is described.

A transmitter STA may transmit a Request To Send (RTS) frame to adestination STA before transmitting an HEW PPDU. Further, thetransmitter STA may receive a Clear To Send (CTS) frame from thedestination STA as a response. A transmission bandwidth of the HEW PPDUmay be determined using a bandwidth signal through RTS/CTS exchangebetween the transmitter STA and the destination STA.

If the transmitter STA performs Clear Channel Assessment (CCA) and if itis determined that a 40 MHz channel is idle, the RTS frame istransmitted through the 40 MHz channel. The destination STA receives theRTS frame only in the 20 MHz channel if only the 20 MHz channel is idle,and the destination STA responds with the CTS frame to the transmitterSTA in the 20 MHz channel. Since the transmitter STA transmits the RTSframe through the 40 MHz channel but receives the CTS frame as aresponse only in the 20 MHz channel, a transmission bandwidth of an HEWPPDU may be less than or equal to a channel bandwidth in which aresponse is received using the CTS frame.

FIG. 9 shows bandwidth signaling according to an embodiment of thepresent invention. An STA1 is a transmitter STA, and an STA2 is adestination STA.

Before transmitting an HEW PPDU, the STA1 transmits an RTS frame to theSTA2, and receives a CTS frame from the STA2. The STA1 performs CCA, andsince it is determined that an 80 MHz channel is idle, transmits the RTSframe through the 80 MHz channel in unit of 20 MHz channel in aduplicated manner. That is, four 20 MHz RTS frames (i.e., one 20 MHz RTSframe and three duplicated RTS frames) are transmitted at an 80 MHzband. For the purpose of decreasing a PAPR of an STA for transmittingthe RTS frame, a value of {1, −1, −1, −1} may be multiplied each 20 MHzchannel.

In the STA2, only a 40 MHz channel is idle and thus the RTS frame isreceived only through the 40 MHz channel. The STA2 responds with the CTSframe to the STA1 in the 40 MHz channel.

Although the STA1 transmits the RTS frame through the 80 MHz channel,the CTS frame is received only through the 40 MHz channel. Therefore, atransmission bandwidth of an HEW PPDU transmitted at a later time may beset to a 40 MHz channel bandwidth at which a response is received usingthe CTS frame.

The CTS frame may also be transmitted in a duplicated manner in unit of20 MHz. For the purpose of decreasing a PAPR of the STA2 fortransmitting a plurality of CTS frames, a value of {1, j} may bemultiplied each 20 MHz channel.

The HEW PPDU can be transmitted simultaneously to a plurality ofdestination STAs by a transmitter STA by independently dividing achannel. In FIG. 9, as to a PSDU transmitted by the STA1, one PSDU istransmitted to the STA2 by using the lowermost 20 MHz channel, and atthe same time, another PSDU is transmitted to an STA3 by using a 20 MHzchannel thereon. However, optionally, it is also possible that thetransmitter STA, i.e., STA1, performs transmission to only onedestination STA without having to independently divide all availablechannels.

When the HEW PPDU is simultaneously transmitted to a plurality ofdestination STAs by independently dividing a channel, a channelbandwidth of each PSDU addressed to each destination STA may be limitedto be less than or equal to a channel bandwidth in which a response isreceived using the CTS frame. Also, a sum of channel bandwidths of allPSDUs in the HEW PPDU may be limited to be less than or equal to achannel bandwidth in which a response is received using the CTS frame.After exchanging RTS/CTS frame, the HEW PPDU being simultaneouslytransmitted to a plurality of destination STAs may have a PSDU addressedto a STA responding a CTS frame. In FIG. 9, because the STA2 respondswith the CTS frame, the PSDU addressed to the STA2 is included in theHEW PPDU.

A phase rotation sequence of a HEW PPDU may be determined based on atransmission bandwidth of the HEW PPDU. A phase rotation sequence of aHEW PPDU transmitted to either a single destination STA or transmittedto a plurality of destination STAs is same when the transmissionbandwidth of the HEW PPDU is identical. In FIG. 9, an HEW PPDU using 512FFT in a 40 MHz channel is applying the same phase rotation sequence asdescribed in FIG. 6 although the PSDUs of HEW PPDU are addressed to aplurality of destination STAs.

When the HEW PPDU is transmitted at the 2.4 GHz band, RTS/CTS needs tobe transmitted through a non-OFDM frame for compatibility with thelegacy STA supporting IEEE 802.11b/g.

FIG. 10 shows a Direct Sequence Spread Spectrum (DSSS) PPDU format usedin IEEE 802.11b/g.

If an RTS/CTS frame is transmitted with the DSSS PPDU format, a channelbandwidth at which the RTS/CTS frame is transmitted is encoded in an8-bit SERVICE field as follows.

TABLE 2 B0 B1 B2 B3 B4 B5 B6 B7 Reserved Reserved Locked clocksCH_BANDWIDTH_IN_NON_HT DYN_BANDWIDTH_IN_NON_HT Length bit. 0 = not,extension 1 = locked bit

A value CH_BANDWIDTH_IN_NON_HT is included in B4-B5 of the SERVICEfield, and is encoded as follows.

TABLE 3 CH_BANDWIDTH_IN_NON_HT Value CBW5 0 CBW20 1 CBW40 2 CBW80 3

When the transmitter STA transmits an RTS frame, CH_BANDWIDTH_IN_NON_HTis encoded in such a manner that the full channel bandwidth which isused to transmit the RTS frame since it is determined to be currentlyidle has a value of 5 MHz, 20 MHz, 40 MHz, and 80 MHz. When thedestination STA responds with a CTS frame, CH_BANDWIDTH_IN_NON_HT isencoded in such a manner that the full channel bandwidth which is usedto transmit the CTS frame since it is determined to be currently idlehas a value of 5 MHz, 20 MHz, 40 MHz, and 80 MHz.

A flag value ‘DYN_BANDWIDTH_IN NON_HT’ is included in B6 of the SERVICEfield, and indicates whether a dynamic channel bandwidth signal is usedthrough RTS/CTS. If the dynamic channel bandwidth signal is used, itimplies that a DATA frame of an HEW PPDU can be transmitted with achannel bandwidth less than the full channel bandwidth of an RTS firsttransmitted by the transmitter STA. Therefore, when the destination STAresponds with the CTS frame, it is possible to respond with the CTS evenif a channel bandwidth determined to be currently idle is less than thefull channel bandwidth of the RTS. However, if the dynamic channelbandwidth signal is not used, it implies that the DATA frame of the HEWPPDU cannot be transmitted with a channel bandwidth less than the fullchannel bandwidth of the RTS first transmitted by the transmitter STA.Therefore, it implies that, when the destination STA responds with theCTS frame, it is not allowed to respond with the CTS frame if thechannel bandwidth determined to be currently idle is less than the fullchannel bandwidth of the RTS.

Meanwhile, control frames (e.g., RTS frame, CTS frame, ACK frame, BlockACK frame, CF-END frame) include a duration field which plays a role ofprotecting frames to be transmitted subsequently. For example, theduration field may indicate a remaining Transmission Opportunity (TXOP)duration or may indicate an estimated time required for the transmissionof the following frame. If a receiving STA is not a destination STA ofthe received frame, the receiving STA can set a Network AllocationVector (NAV) during the time indicated by the duration field. When theNAV is set, the STA considers the channel is busy and does not accessthe channel.

In general, control frames (e.g., RTS frame, CTS frame, ACK frame, BlockACK frame, CF-END frame) are transmitted with a legacy PPDU format sothat the control frames can be received by all STAs. However, if thecontrol frames are transmitted through an HEW PPDU, a GROUP ID field andPARTIAL AID field of an HEW SIGA are respectively set to 63 and 0 in theHEW PPDU. In case of frames other than the control frames, the GROUP IDand PARTIAL AID field are configured as shown in the following table:

TABLE 4 Group Condition ID Partial AID Addressed to AP  0 BSSID[39:47]Sent by an AP and 63 (dec(AID[0:8]) + dec(BSSID[44:47] addressed to aSTA XOR BSSID[40:43]) × 2⁵) mod 2⁹ associated with that AP

where XOR is a bitwise exclusive OR operation, mod X indicates theX-modulo operation, dec(A[b:c]) is the cast to decimal operator where bis scaled by 2⁰ and c by 2 ^(c-b).

A basic service set (BSS) may include a set of STAs that havesuccessfully synchronized with an AP. A basic service set identifier(BSSID) is a 48 bits identifier of a corresponding BSS.

The reason of setting the GROUP ID field and PARTIAL AID field of theHEW SIGA respectively to 63 and 0 with respect to the control frames isto allow STAs other than a destination STA to receive a correspondingcontrol frame and to correctly configure an NAV through a durationfield. The receiving station can be, for example, a destination stationin the present example embodiments disclosed herein.

In the control frame (e.g., RTS frame, CTS frame, ACK frame, Block ACKframe, CF-END frame, etc.) transmitted to an AP, the GROUP ID andPARTIAL_AID of the HEW SIGA are respectively set to 63 and 0 other than0 and BSSID[39:47]. This means that, even if the GROUP ID and PARTIALAID values of the frame received by the AP are respectively set to 63and 0 other than 0 and BSSID[39:47], the AP must process the frameinstead of filtering the frame out. In the control frame (e.g., RTSframe, CTS frame, ACK frame, Block ACK frame, CF-END frame, etc.)transmitted to the AP, the GROUP ID and PARTIAL AID field of the HEWSIGA are respectively set to 63 and 0 other than 0 and BSSID[39:47].This also means that, even if a GROUP_ID and PARTIAL_AID field value ofa frame received by an STA are respectively to 63 and 0 other than 63and (dec(AID[0:8])+dec(BSSID[44:47] XOR BSSID[40:43])×2⁵) mod 2⁹, theSTA must process the frame instead of filtering the frame out.

A COLOR value is used for identifying the BSS, and the number of bitsthereof is less than that of a BSSID. For example, the BSSID may be 48bits, whereas the COLOR value may be 3 bits. The BSSID has the sameformat as a MAC address, whereas the COLOR value is any value reportedin advance by the AP to the STA.

A COLOR field indicating the COLOR value may be included in an HEW-SIGA.In order to report whether the COLOR field is present, the HEW-SIGA mayfurther include a COLOR indication field. For example, if the COLORindication field is set to 0, it indicates that the COLOR field ispresent in the HEW-SIGA. If the COLOR indication field is set to 1, itindicates that the COLOR field is not present in the HEW-SIGA.

If the COLOR field is included as an identifier for identifying a BSS inthe HEW SIGA, the COLOR field may be set to a specific value such as 0.

If a received frame has the COLOR field which is set to a specific valuesuch as 0, this implies that the received frame needs not to be filteredout but to be processed.

As described above, in the HEW PPDU format, the transmitter STA canperform simultaneous transmission to the plurality of destination STAsby independently divided channels. In addition, for the purpose ofbandwidth signaling through an RTS/CTS frame, the RTS/CTS frame may betransmitted as a PPDU format in each subchannel.

FIG. 11 shows data transmission according to an embodiment of thepresent invention.

This is a case where a transmitter STA transmits a PPDU to a pluralityof destination STAs by independently dividing a channel. The transmitterSTA STA1 can perform simultaneously transmission to the plurality ofdestination STAs by independently dividing the channel. This is adownlink OFDMA case if the STA1 is regarded as an AP.

The STA1 performs a back-off procedure in a primary channel (a channelshown in a lowermost portion in FIG. 11), and thereafter transmits aPPDU. The transmitter STA needs to perform transmission to the pluralityof destination STAs, i.e., the STAs 2, 3, 4, and 5, during sametransmission time. The PPDU may include a plurality of PSDUs for theplurality of destination STAs. In order to make PSDUs have sametransmission time, the PPDU is generated as an aggregated medium accesscontrol (MAC) protocol data unit (A-MPDU). A Null A-MPDU having a lengthof 0 is attached to adjust the transmission time to be equal to eachother.

After the plurality of destination STAs receive the PPDU, eachdestination STA transmits a response frame (e.g., block ACK) to theSTA1. The response frame transmitted by each destination STA to the STA1must also be transmitted with same transmission time.

There may be two options to transmit the response frame. In a firstoption, the plurality of destination STAs simultaneously transmit theresponse frame to the transmitter STA by independently dividing achannel. In a second option, each destination STA transmits the responseframe sequentially to the transmitter STA by using a full channelbandwidth. In order to support the sequential response frametransmission from each destination STA, the transmitter STA transmits aresponse request frame such as a block ACK request.

When the destination STA transmits the response frame to the STA1, atransmission bandwidth of the response frame may be less than atransmission bandwidth of the PPDU transmitted by the STA1. Atransmission bandwidth for subsequent PPDU transmission of thetransmitter STA may be less than or equal to the transmission bandwidthof the response frame.

As shown in FIG. 11, duration fields of MPDUs in an A-MPDU may be set tosame values. Comparing A-MPDUs to be transmitted to STA4 and STA5,duration fields of MPDUs constituting an A-MPDU for the STA4 are set to‘A’. In addition, duration fields of MPDUs constituting an A-MPDU forthe STA5 are also set to ‘A’. That is, in a case where the transmitterSTA performs simultaneous transmission to the plurality of destinationSTAs by independently dividing a channel, the duration fields of theMPDUs have the same values in two aspects, i.e., in PPDUs to betransmitted to different destination STAs and in PPDUs to be transmittedto one destination STA. Also those two aspects can be interpreted asfollows: in PPDUs to be transmitted on different channels and in PPDUsto be transmitted on same channel.

If a STA receives PPDUs transmitted in different channels and the PPDUshave the same TA field in a MAC header, this is a case where thetransmitter STA performs simultaneous transmission to the plurality ofdestination STAs by independently dividing the channel. If an erroroccurs in a PPDU in a certain channel, a duration field of the erroneousPPDU cannot be known. The STA may obtain a duration field of theerroneous PPDU from a duration field of another PPDU currently receivedin a different channel. Therefore, in this case, this means that a DCFInterframe Space (DIFS) can be used without having to use an ExtendedInter-Frame Space (EIFS) when a corresponding STA performs a channelaccess procedure.

In a case where an error occurs in a frame received during a channelaccess procedure of an STA and thus a duration field cannot be read, theEIFS is a value used for channel access deferring by providing aninter-frame space as a value greater than or equal to a transmissiontime of an ACK control frame for the purpose of protecting an ACKcontrol frame which can be transmitted at a later time. On the otherhand, the DIFS implies a minimum inter-frame space provided in a channelaccess procedure in normal data frame transmission.

FIG. 12 shows data transmission according to another embodiment of thepresent invention.

A plurality of transmitter STAs perform simultaneous transmission to onedestination STA by independently dividing a channel, which is identicalto an uplink OFDMA case if an STA1 is regarded as an AP.

An STA2 performs a back-off procedure in a primary channel (a channelshown in a lowermost portion in FIG. 12), and thereafter transmits aPPDU. In this case, the transmitter STAs correspond to STAs 3, 4, and 5,and perform simultaneous transmission by independently dividing eachchannel. The plurality of transmitter STAs must perform simultaneoustransmission to one designation STA, i.e., the STA1, during sametransmission time. STA2, 3, 4 and 5 may transmit a plurality of PPDUs toone destination STA. In order to make PPDUs have same transmission time,respective PPDU is generated as an A-MPDU. A Null A-MPDU having a lengthof 0 is attached to adjust the transmission time to be equal to eachother.

After one destination STA receives the PPDUs, the destination STAtransmits a response frame (e.g., block ACK) to each transmitter STA.The response frames are transmitted by the destination STA to eachtransmitter STA with a same transmission time.

There may be two options to transmit the response frames. In a firstoption, the destination STA simultaneously transmits the response framesto the plurality of transmitter STAs by independently dividing thechannel. In a second option, the destination STA configures one blockACK frame for the plurality of transmitter STAs and transmits the framein a broadcast manner by using a full channel bandwidth.

When the destination STA transmits the response frame to the transmitterSTAs, a transmission bandwidth of the response frame may be less than orequal to a sum of transmission bandwidths of the PPDUs transmitted bythe transmitter STAs. A transmission bandwidth for subsequent PPDUtransmission of the transmitter STAs may be less than or equal to thetransmission bandwidth of the response frame.

As shown in FIG. 12, the duration fields of MPDUs transmitted bytransmitter STAs may be set to the same values. Comparing A-MPDUstransmitted by STAs 4 and 5, duration fields of MPDUs constituting anA-MPDU transmitted by the STA4 are set to ‘A’. In addition, durationfields of MPDUs constituting an A-MPDU transmitted by the STAS are alsoset to ‘A’. That is, in a case where the plurality of transmitter STAsperform simultaneous transmission to one destination STA byindependently dividing a channel, the duration fields of the MPDUs havethe same value in two aspects, i.e., in PPDUs transmitted by differenttransmitter STAs and in PPDUs transmitted by one transmitter STA. Alsothose two aspects can be interpreted as follow: in PPDUs transmitted ondifferent channels and in PPDUs transmitted on same channel.

If a STA receives PPDUs transmitted in different channels and the PPDUshave the same RA field in a MAC header or the same partial AID in a PLCPheader, this is a case where the plurality of transmitter STAs performsimultaneous transmission to one destination STA by independentlydividing the channel. If an error occurs in a PPDU in a certain channel,a duration field of the erroneous PPDU cannot be known. The STA mayobtain a duration field value of the erroneous PPDU from a durationfield of another PPDU currently received in a different channel.Therefore, in this case, this means that a DIFS can be used withouthaving to use an EIFS when a corresponding STA performs a channel accessprocedure.

Now, a channel access mechanism is described according to an embodimentof the present invention. It is proposed to adjust a Clear ChannelAssessment (CCA) sensitivity level.

A basic service set (BSS) may include a set of STAs that havesuccessfully synchronized with an AP. A basic service set identifier(BSSID) is a 48 bits identifier of a corresponding BSS. An overlappingbasic service set (OBSS) may be a BSS operating on the same channel asthe STA's BSS. The OBSS is one example of different BSS with the STA'sBSS.

When an STA performs the channel access mechanism, it is firstdetermined whether a channel state of a 20 MHz primary channel isidle/busy. If the channel state is idle, a frame is directly transmittedafter a Distributed Inter-Frame Space (DIFS) elapses. Otherwise, if thechannel state is busy, the frame is transmitted after performing aback-off procedure.

In the back-off procedure, a STA selects any random number in the rangebetween 0 to Contention Window (CW) and sets the number as a back-offtimer. If a channel is idle during a back-off slot time, the back-offtimer is decremented by 1. When the back-off timer reaches 0, the STAtransmits the frame.

In the channel access mechanism, a PHY-CCA. indication primitive isutilized as a means for determining whether the channel state is idle orbusy. When the channel state is idle or busy in a Physical layer (PHY)entity, the PHY-CCA.indication primitive is called out and stateinformation is delivered from the PHY entity to a MAC entity.

According to the section 7.3.5.12 of IEEE P802.11-REVmc/D2.0,PHY-CCA.indication is described as follows.

-   -   7.3.5.12 PHY-CCA.indication    -   7.3.5.12.1 Function    -   This primitive is an indication by the PHY to the local MAC        entity of the current state of the medium and to provide        observed IPI values when IPI reporting is turned on.    -   7.3.5.12.2 Semantics of the Service Primitive    -   The primitive provides the following parameters:

PHY-CCA.indication(     STATE,     IPI-REPORT,     channel-list     )

-   -   The STATE parameter can be one of two values: BUSY or IDLE. The        parameter value is BUSY if the assessment of the channel(s) by        the PHY determines that the channel(s) are not available.        Otherwise, the value of the parameter is IDLE.    -   The IPI-REPORT parameter is present if        dotl1RadioMeasurementActivated is true and if IPI reporting has        been turned on by the IPI-STATE parameter. The IPI-REPORT        parameter provides a set of IPI values for a time interval. The        set of IPI values may be used by the MAC sublayer for Radio        Measurement purposes. The set of IPI values are recent values        observed by the PHY entity since the generation of the most        recent PHYTXEND.confirm, PHY-RXEND.indication,        PHY-CCARESET.confirm, or PHY-CCA. indication primitive,        whichever occurred latest.    -   When STATE is IDLE or when, for the type of PHY in operation,        CCA is determined by a single channel, the channel-list        parameter is absent. Otherwise, it carries a set indicating        which channels are busy. The channel-list parameter in a        PHY-CCA. indication primitive generated by a HEW STA contains at        most a single element. The channel-list parameter element        defines the members of this set.

In the PHY-CCA.indication primitive, the channel state is determined tobe busy in the following conditions.

TABLE 5 Operating Channel Width Channel Busy Conditions  20 MHz, 40 MHz,80 MHz, The start of a 20 MHz non-HT or HT or VHT PPDU in the 160 MHz,or 80 + 80 MHz primary 20 MHz channel at or above −82 dBm. The start ofa 20 MHz HEW PPDU in the primary 20 MHz channel at or above −82+Δ, dBm. 40 MHz, 80 MHz, 160 MHz, The start of a 40 MHz non-HT duplicate or HTor VHT PPDU or 80 + 80 MHz in the primary 40 MHz channel at or above −79dBm. The start of a 40 MHz HEW PPDU in the primary 20 MHz channel at orabove −79+Δ, dBm.  80 MHz, 160 MHz, or The start of an 80 MHz non-HTduplicate or VHT PPDU in 80 + 80 MHz the primary 80 MHz channel at orabove −76 dBm. The start of an 80 MHz HEW PPDU in the primary 20 MHzchannel at or above −76+Δ, dBm. 160 MHz or 80 + 80 MHz The start of a160 MHz or 80 + 80 MHz non-HT duplicate or VHT PPDU at or above −73 dBm.The start of a 160 MHz HEW PPDU in the primary 20 MHz channel at orabove −73+Δ, dBm.

In the Table above, when an adjusting parameter Δ is a positive number,this means that a threshold value for determining whether a channelstate of an HEW STA which receives an HEW PPDU is idle/busy is greaterthan a threshold value for determining this when the conventional legacyPPDU (e.g., a non-HT/HT/VHT PPDU) is received. That is, a thresholdvalue for determining that a channel state is busy as to a framereceived from other BSS (i.e. an OBSS AP/STA) may be set to greater thana threshold value for same BSS. The greater threshold value may resultin a decrease in a service coverage of corresponding OBSS transmission.

In order to configure a small BSS in which a service coverage of the BSSis decreased, the adjusting parameter Δ may be set to a value greaterthan or equal to 3.

In order to adjust a CCA sensitivity level as such, there is a need fora method capable of identifying a BSS of received frames since framescan be received from various kinds of BSSs. That is, there is a need toidentify whether a currently received frame is transmitted by a STAbelonging to the different BSS (i.e. the OBSS AP/STA) or a STA belongingto the same BSS. For example, this is because the determining of thechannel state to be idle by increasing the CCA sensitivity level mayeventually result in a collision and thus may cause deterioration inthroughput performance, if the currently received frame is transmittedby a different STA belonging to the same BSS to an AP or is transmittedby the AP to the different STA belonging to the same BSS.

Increasing the CCA sensitivity has a purpose of improving throughputperformance by making frequent simultaneous transmissions and using aModulation Coding Scheme (MCS) with a high tolerant to an interferencecaused from the OBSS AP/STA.

A STA becomes a member of a BSS for an AP by establishing a connectionwith the AP. The STA can receive information about a BSSID from the AP.To perform the CCA, the STA can adjust its CCA sensitivity level. If areceived PPDU is transmitted from same BSS with the BSS of the STA, theCCA sensitivity level may be set to a first threshold for determiningwhether a channel state of a received PPDU is idle/busy. If a receivedPPDU is transmitted from different BSS with the BSS of the STA, the CCAsensitivity level may be set to a second threshold for determiningwhether a channel state of a received PPDU is idle/busy. The secondthreshold is different from the first threshold. The second thresholdmay be greater than the first threshold. The second threshold may be 3dBm or more greater than the first threshold.

When a STA is failed to identify whether a currently received frame istransmitted by a STA belonging to the different BSS or a STA belongingto the same BSS (e.g., PHY header error and non-HT or HT PPDUreception), the CCA sensitivity level may be set to a first thresholdfor determining whether a channel state of a received PPDU is idle/busy.

An embodiment of the present invention proposes to define a COLOR fieldto identify a BSS. The COLOR field is used for identifying the BSS, andthe number of bits thereof is less than that of a BSSID. For example,the BSSID may be 48 bits, whereas the COLOR bit may be 3 bits. The BSSIDhas the same format as a MAC address, whereas the COLOR field is anyvalue reported in advance by the AP to the STA.

FIG. 13 shows an HEW PPDU format according to an embodiment of thepresent invention.

A COLOR field indicating a COLOR value may be included in an HEW-SIGA.In order to report whether the COLOR field is present, the HEW-SIGA mayfurther include a COLOR indication field. For example, if the COLORindication field is set to 0, it indicates that the COLOR field ispresent in the HEW-SIGA. If the COLOR indication field is set to 1, itindicates that the COLOR field is not present in the HEW-SIGA.

The COLOR value may be allocated by an AP to each STA. Informationregarding the allocated COLOR value may be included in a beacon frame, aprobe response frame, and an association response frame.

A group ID and a partial AID may be utilized as a method of indicatingCOLOR bits:

TABLE 6 Group Condition ID Partial AID Addressed to AP  0 BSSID[39:47]Sent by an AP and 63 (dec(AID[0:8]) + dec(BSSID[44:47] addressed to aSTA XOR BSSID[40:43]) × 2⁵) mod 2⁹ associated with that APwhere XOR is a bitwise exclusive OR operation, mod X indicates theX-modulo operation, dec(A[b:c]) is the cast to decimal operator where bis scaled by 2⁰ and c by 2^(c-b).

An association identifier (AID) represents the 16-bit identifierassigned by an AP during association. A partial AID is a non-unique9-bit STA identifier and is obtained from the AID.

When the STA transmits a frame to the AP, the group ID has a value of 0and the partial AID has a value of BSSID[39:47]. In doing so, as to theframe addressed to the AP, it is possible to identify whether the frameis transmitted from an STA belonging to the same BSS or an STA belongingto the different BSS. Therefore, in case of an uplink frame, it ispossible to reuse the partial AID on the behalf of the COLOR bits.

In case of a frame transmitted by the AP to the STA, the group ID is 63,and the partial AID may be determined as follows.(dec(AID[0:8])+dec(BSSID[44:47]XOR BSSID[40:43])×2⁵) mod 2⁹.  [Equation6]

The partial AID has a value between 1 to 511. In this case, it is notpossible to identify whether a corresponding frame is transmitted by anAP belonging to the same BSS or an AP belonging to the different BSS.

Therefore, in case of a downlink unicast frame, the partial AID cannotbe reused with the COLOR bits, and thus the HEW-SIGA needs to have theCOLOR field.

If an HEW AP overhears a frame having a value of a group ID 63 and apartial AID in the range of 1 to 511, the HEW AP can acknowledge whetherthe frame is transmitted by an OBSS AP to a different OBSS STA or theframe is transmitted directly between STAs belonging to the same BSS. Inother words, if an HEW STA overhears a frame having the group ID 63 andthe partial AID in the range of 1 to 511, the HEW STA cannot knowwhether the frame is transmitted by the AP belonging to the same BSS orby the OBSS AP. However, the HEW AP can confirm that the frame istransmitted from the OBSS STA if it is known that STAs to whichdirection communication (e.g., a Direct Link Setup (DLS) or a TunneledDirect Link Setup (TDLS)) was established in a BSS and if a partial AIDof the received frame is not identical to a partial AID of a peer STA towhich direct communication was established. In addition, in this case, achannel access mechanism may be continued by increasing a CCAsensitivity level.

However, if the partial AID of the received frame is identical to thepartial AID of the peer STA, the HEW AP may follow one of two proceduresas follows.

First, if the channel access mechanism is continued by increasing theCCA sensitivity level but a back-off timer expires, the HEW AP maytransmit a frame to another STA other than STAs which have the partialAID of the received frame.

Second, the channel access mechanism may be deferred until expecteddirect communication is completed.

A COLOR value of a BSS may be delivered to the HEW STA through a beaconframe, a probe response frame, and a (re)-association response frame.Alternatively, the HEW STA may overhear any frame belonging to the BSSand may extract the COLOR value from the overheard frame. If a STA knowsthe COLOR value of the BSS, the STA may set the COLOR value in theHEW-SIGA for a frame to be transmitted to another STA belonging to theBSS.

Least Significant Bit (LSB) 3 bits of the partial AID or MostSignificant Bit (MSB) 3 bits thereof may be utilized as the COLOR value.In this case, as one embodiment for a legacy STA, for example, a HEW APcan calculate a partial AID in the same manner as shown in Equation 6when the HEW AP sends a frame to a VHT STA.

The HEW AP may allocate an AID of the STA such that LSB 3 bits or MSB 3bits have the same COLOR value. The AP may send a PPDU1 with the COLORfield and a COLOR indication field that is set to 0. The AP may send toa VHT STA a PPDU2 with a COLOR indication field that is set to 1. A HEWSTA which overhears the PPDU2 does not acquire any COLOR informationfrom the PPDU2. This is because the AP may allocate an AID of a legacySTA in a conventional manner without considering the COLOR value.

In case of a broadcast/multicast frame transmitted by the AP to allSTAs, a group ID is set to 63 and a partial AID is set to 0. Since thegroup ID and the partial AID have the same value irrespective of a BSS,it is not possible to identify whether the frame is transmitted from anAP belonging to the same BSS or an OBSS AP. Accordingly, in case of adownlink broadcast/multicast frame, a partial AID cannot be reused withCOLOR bits, and thus the HEW-SIGA needs to have COLOR 3 bits.

However, this may be limited to the HEW STA. If the HEW AP overhears aframe having a group ID 63 and a partial AID 0, it can be confirmed thatthe frame is transmitted from the OBSS AP. In other words, if the HEWSTA overhears the frame having the group ID 63 and the partial AID 0,the HEW STA cannot know whether the frame is transmitted from the APbelonging to the same BSS or the OBSS AP. However, the HEW AP candetermine this, and thus a channel access mechanism can be continued byincreasing a CCA sensitivity level.

When it is known that the currently received frame is transmitted fromdifferent BSS (i.e. the OBSS AP/STA), the HEW AP may report such a factto its HEW STA. For this, the HEW AP may transmit an OBSS announcementcontrol frame to HEW STAs belonging to the BSS of the HEW AP.

The following table shows a format of the OBSS announcement controlframe. Field names and bit numbers are exemplary purpose only.

TABLE 7 Frame Control Duration RA TA (BSSID) FCS 2 octets 2 octets 2octets 2 octets 2 octets

The duration field may be set to a value obtained by subtracting a delaytime consumed in a channel access process after the HEW AP recognizesOBSS transmission from a transmission time of corresponding OBSStransmission.

The RA field may be set to a broadcast MAC address or individual STA MACaddress. If corresponding OBSS transmission is reported to a specificSTA in order to continue a channel access mechanism by increasing a CCAsensitivity level, the MAC address of the specific STA may be includedin the RA field. Otherwise, if the corresponding OBSS transmission isreported to all STAs belonging to the BSS in order to continue thechannel access mechanism by increasing the CCA sensitivity level, thebroadcast MAC address may be included in the RA field.

The TA field may set to a BSSID of an HEW AP for transmitting the OBSSannouncement control frame.

When the HEW STA receives the OBSS announcement control frame, the HEWSTA may determines whether the OBSS announcement control frame istransmitted by an HEW AP associated with the HEW STA by using the TAfield. If the BSSID in the TA field is same as a BSSID of the HEW STA,the HEW STA may continue to perform the channel access mechanism byincreasing the CCA sensitivity level during an interval indicated by theduration field if the RA field is matched to a MAC address of the HEWSTA or broadcast MAC address.

If the AP transmits a MU-MIMO frame, a group ID has a value in the rangeof 1 to 62. This implies that the MU-MIMO frame does not include thepartial AID field. Since the group ID may be set to any valueirrespective of a BSS, the group ID cannot be used to identify whether acorresponding frame is transmitted by an AP belonging to the same BSS oran OBSS AP. Accordingly, in case of a MU-MIMO frame, HEW-SIG-A needs tohave COLOR 3 bits.

If the HEW AP overhears a frame having a group ID value in the range of1 to 62, the HEW AP can confirm that the MU-MIMO frame is transmittedfrom the OBSS AP. Since the HEW AP can determine whether the receivedframe is transmitted from same BSS or different BSS, the HEW AP cancontinue to perform the channel access mechanism by increasing the CCAsensitivity level.

If bits for HEW-SIG-A field are not sufficient to define the 3-bit COLORfield, a part of 12-bit N_(sTs) field may be set to the 3-bit COLORfield. The N_(sTs) field indicates the number of space time streamstransmitted to each of up to 4 STAs. For each STA, 3 bits indicate 0space time streams, 1 space time stream, 2 space time streams, 3 spacetime streams, and 4 space time streams. However, in order to define theCOLOR field, the number of destination STAs of the MU-MIMO frame may belimited to up to 3, and subsequently, last 3 bits of N_(STS) field maybe used as the COLOR field. The COLOR indication field may indicatewhether the COLOR field is present in the HEW-SIG-A field. When the VHTAP transmits the MU-MIMO frame to the VHT STA, the COLOR indicationfield value is set to 1, and thus the HEW STA which overhears the MU-MMOframe does not acquire any COLOR information from the Ns_(STS) field ofthe MU-MIMO frame. This is because the VHT AP corresponding to thelegacy AP allocates the group ID and the N_(STS) to the STA in theconventional way without considering the COLOR value.

Now, a method of managing a Multiple Basic Service Set (BSS) will bedescribed.

A BSS may include a set of STAs that have successfully synchronized withan AP. A basic service set identifier (BSSID) is a 48 bits identifier ofa corresponding BSS. An overlapping basic service set (OBSS) may be aBSS operating on the same channel as the STA's BSS. The OBSS is oneexample of different BSS with the STA's BSS.

When a virtual AP is configured in a WLAN environment in which aplurality of users operate in a dense area, an STA can be managed moreeffectively.

The virtual AP is used in a scheme in which several virtual APs areimplemented by physically one AP, so that STAs can selectively connectwith one of the virtual APs.

For this, the AP may transmit a Beacon frame or Probe response framehaving a Multiple BSSID element.

FIG. 14 shows a Multiple BSSID element format according to an embodimentof the present invention.

The Max BSSID Indicator field contains a value assigned to n, where 2nis the maximum number of BSSIDs in the multiple BSSID set, including thereference BSSID. The actual number of BSSIDs in the multiple BSSID setis not explicitly signaled. The BSSID(i) value corresponding to the i-thBSSID in the multiple BSSID set is derived from a reference BSSID (REFBSSID) as follows:BSSID(i)=BSSID_A OR BSSID_B,where:

BSSID_A is a BSSID with (48−n) MSBs equal to the (48−n) MSBs of theREF_BSSID and n LSBs equal to 0,

BSSID_B is a BSSID with (48−n) MSBs equal to 0 and n LSBs equal to [(nLSBs of REF_BSSID)+i] mod 2n.

Through the Max BSSID indicator, the maximum number of supportablevirtual APs and BSSID(i) connected to each virtual AP can be calculated.

FIG. 15 shows a Multiple BSSID index element format according to anembodiment of the present invention.

A Beacon frame or a Probe Response frame may include the Multiple BSSIDindex element in addition to a Multiple BSSID element. The MultipleBSSID index element configures a DTIM Period and a DTIM Count for eachvirtual AP. If the Multiple BSSID index element is included in the ProbeResponse frame, the DTIM Period and the DTIM Count may be omitted.

The BSSID Index field is a value between 1 and 2^(n)−1 that identifiesthe non-transmitted BSSID, where n is a nonzero, positive integer value.

The DTIM Period field is the DTIM period field for the BSSID. This fieldis not present when the Multiple BSSID-Index element is included in theProbe Response frame.

The DTIM Count field is the DTIM count field for the BSSID. This fieldis not present when the Multiple BSSID-Index element is included in theProbe Response frame.

Another feature of the virtual AP is that an Association ID (AID) of anSTA connected to each virtual AP and a Traffic Indication Map (TIM) aredelivered through a single information element. That is, this impliesthat different values are allocated to STAs A and B connected to BSSIDs1 and 2. Although being connected to APs each having a different BSSID,the STAs are connected to a physically single AP. The single APinternally manages the STAs, and thus AID ranges for the STAs areselected in the common range.

An STA which acknowledges that a virtual AP is supported through theMultiple BSSID element and the Multiple BSSID index element calculates aBSSID(i) (i.e., BSSID(i)=BSSID_A OR BSSID_B) of the virtual AP to whichthe STA intends to connect, and thereafter performs a management accessand data transmission/reception procedure by using the calculatedBSSID(i).

When a requesting STA transmits an Association Request frame thatincludes address fields, for example, a Receiver Address (RA) (Addresslfield) and a Transmitter Address (TA) (Address2 field), the BSSID(i)value is set to the RA (Address1 field) and the requesting STA's MACaddress value is set to the TA (Address2 field). A virtual AP whichreceives the Association Request frame responds with an AssociationResponse frame to the requesting STA. The Association Response frame mayinclude information about an AID allocated to the requesting STA.

In order to set an AID as non-zero, the AID may be allocated to the STAin such a manner that the following equation:(dec(AID[0:8])+dec(BSSID[44:47]XOR BSSID[40:43])×2⁵)mod 2⁹.   [Equation7]

The Partial AID is included in a PLCP header and used to identify anintended STA. If a Partial AID of a PPDU currently being received is notidentical to a Partial AID of the STA, the STA may no longer decode thePPDU but may discard the PPDU.

A partial AID of a Multicast/Broadcast frame or a frame transmitted froman unassociated STA is set to 0 for a special purpose. A filteringprocedure is not performed on a PPDU of which a Partial AID value is 0.Therefore, in order to decrease unnecessary overhearing of the STA, anAID obtained from the proposed equation 7 should be a non-zero.

An STA which supports a virtual AP may calculate a BSSID(i) of a virtualAP to which the STA intends to have a connection, and delivers thisimplicitly to the virtual AP through an Association Request frame (e.g.,an RA (Address1 field)). The virtual AP which receives the AssociationRequest frame may allocate an AID to the STA by using an BSSID(i) valueacquired from the Association Request frame such that a Partial AIDvalue obtained by the following equation is non-zero.(dec(AID[0:8])+dec(BSSID(i)[44:47]XOR BSSID(i)[40:43])×2⁵)mod 2⁹.  [Equation 8]

It may be assumed that a Partial AID value is 0 when an AID 100 isassigned to an STA linked to a BSSID(1), and the Partial AID value is 0when an AID 101 is assigned to an STA linked to a BSSID(2). In thiscase, not the AID 100 but the AID 101 is preferably assigned to the STAwhich has a connection to the virtual AP of the BSSID(1), and not theAID 101 but the AID 100 is preferably assigned to the STA which has aconnection to the virtual AP of the BSSID(2).

In case of a downlink unicast frame, the partial AID cannot be reusedwith the COLOR bits, and thus the HEW-SIGA needs to have the COLORfield.

A COLOR value of a BSS may be delivered to a HEW STA through a Beaconframe, a Probe Response frame, and a (Re)-Association response frame.Alternatively, after the HEW STA overhears any frame belonging to theBSS, it may be estimated as a value of a COLOR field included in anHEW-SIGA of the overheared frame. When the COLOR value of the BSS isknown, the HEW STA may set the same COLOR value to an HEW-SIGA for aframe to be transmitted to a peer STA belonging to the BSS. When a HEWSTA does not know a COLOR value of a BSS, the HEW STA is only allowed totransmit a legacy PPDU. If a follow-up frame is to be transmitted in aHEW PPDU, the proceeding PPDU should include the COLOR value of the BSS.For example, a frame eliciting a HEW PPDU transmission may have a COLORvalue of a BSS.

In a virtual AP environment, all COLOR fields for virtual APs may be setto the same value. The virtual AP corresponds to physically a single AP.This is because simultaneous transmission/reception for differentvirtual APs is not possible while one of the virtual APs performstransmission/reception. For example, it is assumed that an STA1 and anSTA2 are connected respectively to a first virtual AP having a BSSID1and a second virtual AP having a BSSID2. PPDU1 transmitted by the STA1and PPDU2 transmitted by the STA2 may have the same COLOR value. PPDU3transmitted by the first virtual AP and PPDU4 transmitted by the secondvirtual AP may also have the same COLOR value. This is because, even ifthe virtual APs have different COLOR values, STAs do not know whetherthe COLOR values is sent from different virtual APs. For example, theSTA1 cannot know whether the COLOR value is sent from the first virtualAP. Thus STA1 cannot distinguish the received COLOR value which is sentfrom the first virtual AP or from the second virtual AP.

Now, a method related to a PPDU transmission and an error recoveryduring a transmission opportunity (TXOP) is described.

A TXOP may be defined as an interval of time during which a STA has theright to initiate frame exchange sequences onto a wireless medium. Anaccess category (AC) may be defined as a label for the common set ofenhanced distributed channel access (EDCA) parameters that are used by astation to contend for the channel in order to transmit medium accesscontrol (MAC) service data units (MSDUs) with certain priorities. The ACrelates to quality-of-service (QoS) requirements.

If a STA transmits one or more PPDUs simultaneously to a plurality ofdestination STAs by independently dividing it for each channel, this maybe called as an OFDMA mode. While operating in the OFDMA mode, the STAcan send one or more PPDUs to the plurality of destination STAs viaplurality of channels as shown in FIGS. 8 and 9.

A subchannel may refer to a transmission unit allocated to eachtransmitter STA in the OFDMA mode. An operating bandwidth can be dividedinto a plurality of subchannels.

If a transmitter STA transmits an HEW PPDU simultaneously to a pluralityof destination STAs by independently dividing it for each channel, theHEW PPDU to be transmitted to each destination STA must have the sameAccess Category. In FIG. 9, a PPDU transmitted by an STA1 to an STA2 anda PPDU transmitted by the STA1 to an STA3 must have the same AccessCategory.

A TXOP Limit is set differently depending on an Access Category of theTXOP. Therefore, this implies that the same TXOP Limit value must beapplied to all PPDUs to be transmitted, if the transmitter STA transmitsthe HEW PPDU simultaneously to the plurality of destination STAs bydividing it for each channel. For this, a Primary Access Category isproposed.

The Primary Access Category may indicate an Access Category of aBack-off timer used by an STA to acquire a TXOP. In FIG. 9, a Back-offtimer is running for each Access Category before an STA1 transmits anRTS frame, and if a Back-off timer corresponding to Access CategoryVoice (AC_VO) reaches 0 among the Back-off timers, the AC_V0 correspondsto the Primary Access Category. If the Primary Access Category isdetermined, the HEW PPDU with the Primary Access Category can only betransmitted.

Since each of the plurality of destination STAs has a different amountof data to be received, HEW PPDUs of different Access Categories can besimultaneously transmitted by independently dividing it for each channelaccording to another embodiment of the present invention. However, inthis case, a TXOP Limit of the corresponding TXOP must be determined bythe Primary Access Category. In FIG. 9, when the Primary Access Categoryis the AC_VO, an Access Category of a PPDU transmitted by the STA1 tothe STA2 must be the AC_VO, and the entire TXOP is restricted by theTXOP Limit of the AC_VO. An Access Category of a PPDU transmitted by theSTA1 to the STA3 may be AC_VI (Video), AC_BE (Best Effort) or AC_BK(Background).

If an available bandwidth of a destination STA is wider than atransmission bandwidth of a transmitter STA which acquires a TXOP, thedestination STA may support simultaneous transmission performed byanother STA by independently dividing it for each channel, in additionto the transmitter STA.

The transmitter STA which has acquired the TXOP through the Back-offmechanism transmits an RTS frame to the destination STA. The bandwidthsignal and Access Category may be included in the RTS frame. On thebasis of the bandwidth and Access Category included in the RTS frame,the destination STA may allow another STA to transmit a data frame forthe destination STA. During the TXOP of the transmitter STA, a channelnot used by the transmitter STA is allowed to be used by another STA. Adestination STA can transmits at least one CTS frames via at least oneidle subchannel. For example, the destination STA may send a first CTSframe via a first subchannel to the transmitter STA and may send asecond CTS frame via a second subchannel to another STA. The transmitterSTA which has received the first CTS frame can transmit a data frame tothe destination STA by utilizing only the first subchannel whichreceives the first CTS frame. The destination STA can also utilize thesecond subchannel to communicate with another STA.

FIG. 16 shows an example of PPDU transmission having an RTS/CTSbandwidth signal.

Before transmitting an HEW PPDU, a transmitter STA, i.e., an STA2,transmits an RTS frame to one destination STA, i.e., an STA1, andreceives a CTS frame as a response from the STA1. The STA2 performsClear Channel Assessment (CCA). The STA2 determines that an 80 MHzchannel is idle, transmits the RTS frame through the 80 MHz channel inunit of 20 MHz channel in a duplicated manner. In order to decrease aPAPR, a phase rotation sequence of {+1, −1, −1, −1} is multiplied overfour 20 MHz channels.

In a case where the destination STA, i.e., the STA1, intends to supportsimultaneous transmission of an HEW PPDU by a plurality of transmitterSTAs by independently dividing it for each channel, a CTS frame may betransmitted to different transmitter STAs for each channel as aresponse. In FIG. 16, it can be seen that the STA1 responds with the CTSframe to the STA2, and at the same time, STA1 sends with a CTS frame toan STA3 in a different channel. Although the CTS frame is simultaneouslytransmitted by being independently divided for each channel with respectto different transmitter STAs, it can be seen that transmission isperformed by multiplying four 20 MHz channels by a phase rotationsequence of {+1, −1, −1, −1}.

The STA2 and STA3 can receive the CTS frames from the STA1 respectively.Respective CTS frame has information about its transmission channel andan Access Category. STA2 and STA3 can send HEW PPDUs to the STA1 viatransmission channels in which corresponding CTS frames are received.

The HEW PPDUs may have same Access Category. In FIG. 16, a HEW PPDU1transmitted by the STA2 to the STA1 and a HEW PPDU2 transmitted by theSTA3 to the STA1 may have the same Access Category. A TXOP Limit is setdifferently depending on an Access Category of the TXOP. Therefore, sameTXOP Limit can be applied to all HEW PPDUs to be transmitted. For this,the aforementioned Primary Access Category may be defined.

The Primary Access Category indicates an Access Category of a Back-offtimer used by a STA to acquire the TXOP. In FIG. 16, a Back-off timer isrunning for each Access Category before an STA1 transmits an RTS frame.If a Back-off timer corresponding to Access Category Voice (AC_VO)reaches 0, the AC_VO corresponds to the Primary Access Category. If thePrimary Access Category is determined, information about the PrimaryAccess Category can be sent to a destination STA. The destination STAcan deliver the Primary Access Category information to the plurality oftransmitter STAs. Accordingly, all PPDUs to be transmitted by theplurality of transmitter STAs can have same Access Category.

Since the plurality of transmitter STAs have a different amount of datato transmit, HEW PPDUs of different Access Categories can besimultaneously transmitted by independently dividing it for each channelaccording to another embodiment of the present invention. However, inthis case, a TXOP Limit of the corresponding TXOP must be determined bythe Primary Access Category. In FIG. 16, when the Primary AccessCategory is the AC_VO, an Access Category of a PPDU transmitted by theSTA1 to the STA2 must be the AC_VO, and the entire TXOP is restricted bythe TXOP Limit of the AC_VO. An Access Category of a PPDU transmitted bythe STA1 to the STA3 may be AC_VI (Video), AC_BE (Best Effort) or AC_BK(Background).

To deliver information about the Primary Access Category to STAs throughan RTS/CTS frame, it is proposed to encode at least one bit of thescrambling sequence with a QoS parameter such as AC_VO, AC_VI, AC_BE,AC_BK.

FIG. 17 shows a scrambling procedure for a data field in a PPDU.

A data field in a PPDU may be scrambled with a length-127frame-synchronous scrambler. The data field includes at least one PDSU.The octets of the PSDU are placed in the transmit serial bit stream, bit0 first and bit 7 last. The 127-bit sequence generated repeatedly by thescrambler shall be (leftmost used first), 00001110 11110010 1100100100000010 00100110 00101110 10110110 00001100 11010100 11100111 1011010000101010 11111010 01010001 10111000 1111111. The same scrambler is usedto scramble transmit data and to descramble receive data. If theparameter CH_BANDWIDTH_IN_NON_HT is not present, the initial state ofthe scrambler may be set to a pseudo-random nonzero state. If theparameter CH_BANDWIDTH_IN_NON_HT is present, the first 7 bits of thescrambling sequence may be set as shown in following table.

TABLE 8 First 7 bits of Scrambling Sequence B0 B1 B2 B3 B4 B5 B6 PrimaryAccess CH_BANDWIDTH_IN_NON_HT Category

Since the first 7 bits of the scrambling sequence are used as ascrambling initial seed, at least 2 bits may be set to a valueindicating the Primary Access Category.

When a Primary Access Category of a corresponding TXOP is known throughan RTS frame, a destination STA can respond with a CTS frame by settingthe Primary Access Category to the same value.

FIG. 18 shows an example of HEW PPDU transmission having an RTS/CTSbandwidth signal.

This is a case where an STA1 responds with a CTS frame to an STA2 and anSTA3, but the STA3 fails to successfully receive the CTS frame. The STA2acquires the TXOP and the STA1 is the destination STA.

If the STA3 fails to successfully receive the CTS frame, the STA3 doesnot transmit a data frame to the STA1. As such, if an error occurs inthe middle of TXOP, the data frame is not transmitted in a channelallocated to the STA3. In order to utilize the channel not used by theSTA3, the STA1 and the STA2 may perform a PCF Interframe Space (PIFS)recovery procedure on all of a primary channel and secondary channels todetermine again a channel bandwidth to be used at a later time.

FIG. 19 shows a PIFS Recovery procedure performed after a frame erroroccurs in the middle of TXOP.

An STA2 acquires TXOP through a Back-off timer of an AC_VO, andsubsequently transmits an RTS frame to an STA1. The STA1 responds with aCTS frame to the STA2 and an STA3 by using different channels. The STA2which has successfully received the CTS frame transmits a PPDU to theSTA1 by using a bandwidth signal included in the CTS frame and a channelthrough which the CTS frame is received. Further, a Block ACK frame isreceived from the STA1 as a response, and a feedback for data frametransmission is received.

However, the STA3 which fails to successfully receive the CTS frame doesnot transmit any PPDU to the STA1.

The STA1 which cannot receive any data frame from the STA3 requests theSTA2, i.e., a TXOP owner, to perform a PIFS Recovery for the purpose ofre-allocating to another STA a channel allocated to the STA3. Such arequest may be signaled through a Block ACK frame transmitted by theSTA1 to the STA2. The STA2 which receives a request for performing thePIFS Recovery from the STA1 may determine whether a channel state is anidle/busy state by performing a CCA process during a PIFS time withrespect to a primary channel and secondary channels.

If the STA1 has a right of the TXOP owner (e.g., the STA1 is a RDresponder in reverse direction protocol), the STA1 may perform the CCAprocess during a PIFS time with respect to a primary channel andsecondary channels. It means that a STA operating in the OFDMA modeperforms the PIFS Recovery for the purpose of re-allocating a channelduring a TXOP, irrespective of the success of the transmitted HEW PPDU.

In FIG. 19, all 80 MHz channels are idle, and the STA2 transmits againan RTS frame in the 80 MHz channel. A destination STA, i.e., the STA1,responds with a CTS frame to the STA2 and the STA3 through respectivedifferent channels, and thus provides the STA3 an opportunity ofsimultaneously transmitting an HEW PPDU independently in a correspondingchannel one more time. In this time, the STA3 which has successfullyreceived the CTS frame also transmits a PPDU to the STA1 by using abandwidth signal included in the CTS frame and a channel through which aCTS frame is received. Further, a Block ACK frame is received from theSTA1 as a response, and a feedback for data frame transmission isreceived.

FIG. 20 shows a Recovery procedure when a frame error occurs.

An STA2 acquires TXOP through a Back-off timer of an AC_VO, andsubsequently transmits RTS frames to an STA1. The STA1 responds with CTSframes to the STA2 and an STA3 by using different channels.

The STA3 which has successfully received the CTS frame transmits a PPDUto the STA1 by using a bandwidth signal included in the CTS frame and achannel through which the CTS frame is received.

However, the STA2 which fails to successfully receive the CTS frame doesnot transmit any PPDU to the STA1. Since the STA2 corresponding to aTXOP owner does not use a primary channel, all STAs including the STA2perform a Back-off mechanism again, and in the above figure, an STA4 cannewly obtain TXOP and transmit RTS frames to the STA1. However, sincethe STA3 is currently transmitting a 40 MHz PPDU, a correspondingchannel state is busy, and thus the RTS frames of the STA4 can betransmitted only through a 40 MHz channel including a primary channel.This is a case where the STA1 receives PPDUs from the STA2 and alsoreceives the RTS frames from the STA4.

In an embodiment, a STA can stop receiving of a frame which is currentlybeing received in secondary channels when a certain frame is received inits primary channel while another frame is received in the secondarychannels. A capture effect is a scheme of immediately stopping receivingof a frame currently being received upon receiving of a signal havingstrength greater by a specific level than or equal to received signalstrength of a frame currently being received in the same channel. Theproposed method extends such a concept of the capture effect, whichmeans that receiving of a certain frame is immediately stop irrespectiveof reception signal strength of a frame currently being received insecondary channels, when the certain frame is received in its primarychannel during the certain frame is received in the secondary channels.

In FIG. 20, the STA1 which has successfully received an RTS frame fromthe STA4 responds with a CTS frame to the STA3, and subsequently, theSTA4 starts to transmit a PPDU to the STA1.

FIG. 21 shows an example of a Medium Access Control (MAC) frame formatbased on the conventional IEEE 802.11.

This frame corresponds to a Protocol Version 0 (PV0) Data frame. The PV0Data frame includes Frame Control, Duration/ID, Address 1 (ReceiverAddress), Address 2 (Transmitter Address), Address 3 (BSSID), SequenceControl, Address 4, Quality-of-Service (QoS) Control, HT Control, MSDU,and Frame Control Sequence (FCS).

The Frame Control field includes Protocol Version, Type, Subtype, To DS,From DS, More Fragment, Retry, Power Management, More Data, ProtectedFrame, and Order.

The Protocol Version may be set to 0 to indicate that a correspondingMAC Protocol Data Unit (MPDU) is a PV0 Data frame. The Type and theSubtype are set to indicate that a corresponding MPDU is a DATA frame,and to specify a detailed type such as QoS data and null data among theData frames. The To DS indicates whether it is transmitted to adistribution system, and the From DS indicates whether it is transmittedfrom the distribution system.

FIG. 22 shows another example of a MAC frame format based on theconventional IEEE 802.11.

This frame corresponds to a Protocol Version 1 (PV1) Data frame. The PV1Data frame includes Frame Control, Address 1 (Receiver Address), Address2 (Transmitter Address), Sequence Control, Address 3, Address 4, MSDU,and FCS.

The Frame Control field of the PV1 Data frame includes Protocol Version,Type, PTID/Subtype, From DS, More Fragment, Power Management, More Data,Protected Frame, End of Service Period, Relayed Frame, and Ack Policy.

The Protocol Version may be set to 1 to indicate that a correspondingMPDU is a PV1 Data frame. The Type and the PTID/Subtype are set toindicate that a corresponding MPDU is a DATA frame, and to specify adetailed type such as QoS data and null data among the Data frames. TheFrom DS indicates whether it is transmitted from a distribution system.According to the From DS field, contents of Address 1 and Address 2 aredetermined.

Table 9 shows contents included in Address 1, Address 2, Address 3, andAddress 4 according to the From DS.

TABLE 9 From DS field Meaning Use 0 A1 contains the MAC address of thereceiver. For frames transmitted by a non-AP STA to an AP. A2 is anShort ID (2 octets) which contains For frames transmitted from a the AIDof the transmitter. non-AP STA to non-AP STA A3 (if present) containsthe MAC address (direct link) of the destination. A4 (if present)contains the MAC address of the source. 1 A1 is an SID (2 octets) whichcontains the AP to non-AP STA AID of the receiver. A2 is the MAC addressof the transmitter. A3 (if present) contains the MAC address of thedestination. A4 (if present) contains the MAC address of the source.

Comparing the PV0 Data frame and the PV1 Data frame, the PV1 Data frameis different from the PV0 Data frame in a sense that fields consideredas being unnecessary, for example, duration/ID and QoS, are excludedfrom a MAC header. Therefore, the PV1 Data frame may be called a shortdata frame. If a size of an MSDU is great, the PV0 data frame ispreferably used, and if the size of the MSDU is small, the PV1 dataframe is preferably used to decrease an overhead for the MAC header.

FIG. 23 shows an A-MPDU format according to an aggregation scheme for anMPDU.

Each of a plurality of MPDUs is configured with an aggregated-MPDU(A-MPDU) subframe and is transmitted by being aggregated with one PPDU.

The A-MPDU subframe includes a 4-octet MPDU delimiter, a MPDU, and a Padoctet.

The MPDU delimiter includes EOF, MPDU length, CRC, and DelimiterSignature.

As an HEW MAC format, it is proposed an aggregation scheme for differenttypes of MPDUs, i.e., a PV0 Data frame and a PV1 Data frame.

FIG. 24 shows a frame format according to an embodiment of the presentinvention.

PV0 and PV1 Data frames are aggregated within one A-MPDU frame.

When aggregating the PV0 and PV1 Data frames, there is a need todistinguish the PV0 Data frame and the PV1 Data frame in order todecrease decoding complexity of a receiver STA. It is not preferable toaggregate the frames in a mixed manner such as the PV0 Data frame, thePV1 Data frame, the PV0 Data frame, and the PV1 Data frame.

The PV0 Data frame and the PV1 Data frame may be aggregatedsequentially. This means that the PV1 Data frame is included in anA-MPDU subframe only after the PV0 Data frame. Since more pieces ofinformation are included in the PV0 Data frame, a load of decodingprocessing may be decreased.

In order to aggregate the PV0 and PV1 Data frames, there are severalrestrictions as follows.

First, as shown in FIG. 24, a Traffic Identifier (TID) value may beidentical for both of a PV0 Data frame and a PV1 Data frame to beaggregated. The PV0 Data frame is encoded through a TID subfield (4bits) of a QoS control field of a MAC header, and the PV1 Data frame isencoded through a PTID subframe (3 bits) of a Frame Control (FC) fieldof the MAC header. The PTID implies a Partial TID, and implies lower 3bits among 4 bits of a TID subfield of a QoS control field. It is shownin FIG. 8 that TID and PTID subfields of the PV0 Data frame and PV1 Dataframe included in the A-MPDU have the same Traffic Identifier (TID)value of ‘B’.

Second, the Address 1 and the Address 2 may indicate the same STA. Incase of the PV0 Data frame, the Address 1 includes a receiver STA MACaddress, and the Address 2 includes a transmitter STA MAC address.However, in case of the PV1 Data frame, although the Address 1 indicatesthe receiver STA and the Address 2 indicates the transmitter STA in thesame manner as described above, a short ID value including an AID isused as one of the Address 1 and the Address 2 according to the From DSsubfield of the Frame Control field.

This implies that the receiver STA indicated by the Address 1 withrespect to the PV0 Data frame and the PV1 Data frame may be identicaleven though contents of the Address 1 are different from each other withrespect to the PV0 Data frame and the PV1 Data frame. Also, this impliesthat the transmitter STA indicated by the Address 2 with respect to thePV0 Data frame and the PV1 Data frame may be identical even thoughcontents of the Address 2 are different from each other with respect tothe PV0 Data frame and the PV1 Data frame.

Third, sequence number values of Sequence Control fields for the PV0Data frame and the PV1 Data frame may be managed as one counter. Thisimplies that the PV0 Data frame with SN1 and the PV1 Data frame with SN2cannot be aggregated together in the same A-MDPU. In other words, thisimplies that, if the PV0 Data frame uses the counter of the SN1, the PV1Data frame is also managed sequentially by using the same counter, i.e.,SN1, so that the frames can be aggregated together in the same A-MDPU.This is because an STA which has received a corresponding A-MPDU assumesthat a sequence number of MPDUs included in the A-MPDU is sequentiallyincreased when transmitting an acknowledgement through Block ACK.

Fourth, all Duration fields of PV0 Data frames constituting the A-MPDUsubframe may be identical. The Duration field is set for the purpose ofprotecting a TXOP duration or a Response PPDU to be transmitted after acorresponding A-MPDU. Other STA does not access the channel during aninterval indicated by the Duration field. In case of the PV1 Data frame,the Duration field may not be included in a MAC header. A Duration fieldvalue in the PV0 Data frame may also indicate a Duration field value ofthe PV1 data frame when the PV1 Data frame is aggregated with the PV1Data frame in the A-MPDU frame.

Fifth, for the PV0 Data frame and PV1 Data frame constituting the A-MPDUsubframe, among Ack Policy fields of corresponding frames, the number of“Normal Ack or Implicit Block Ack requests”, i.e., A-MPDU subframes forrequesting an immediate control response, may not be equal to or greaterthan 2. This is because a collision occurs in a plurality of immediatecontrol responses in this case.

If there is no PV0 Data frame to be transmitted, only the PV1 Data framemay be included in an A-MPDU subframe of an A-MPDU frame. In this case,a TXOP Duration or a Response PPDU to be transmitted at a later timecannot be protected. This is because the Duration field does not existin the MAC header of the PV1 Data frame. In this case, the following PV0Null Data frame can be used.

FIG. 25 shows an A-MPDU format having a PV0 Null Data frame.

A PV0 Null Data frame may include an MPDU not having an MSDU. The PV0Null Data frame may be used to protect a TXOP Duration or a ResponsePPDU to be transmitted at a later time through a duration field of a MACheader.

If only the PV1 Data frame is included as an A-MPDU subframe of theA-MPDU frame or if the PV1 Data frame is transmitted as a single PPDU,there is still a problem in that the TXOP Duration or the Response PPDUto be transmitted at a later time cannot be protected.

As a solution for this, a Response Indication field may be included in aPLCP Header of a corresponding PPDU, for example, in a signal field. Thesignal field may be included in a physical layer preamble of a PPDU. Forexample, the Response Indication field may be in included in L-SIG,HEW-SIGA or HEW-SIGB of an HEW PPDU.

The Response Indication field may indicate a type of an expectedresponse used to protect the response frame. The Response Indicationfield may indicate a type of a Response PPDU to be transmitted after thecorresponding PPDU transmitted at the moment.

The Response Indication field may be set to a value indicating one of NoResponse, Normal Response and Long Response. The No Response indicatesno immediate response that implies that there is no Response PPDU to betransmitted after the corresponding PPDU. The Normal Response indicatesthat an addressed recipient returns an individual control responseframe. The Normal Response may imply that a control response PPDU suchas ACK or Block ACK is to be transmitted starting one Short InterframeSpace (SIFS) after the end of the corresponding PPDU. The Long Responseindicates that an addressed recipient may return a response frame whichis not an individual control response frame. The Long Response may implythat a response PPDU such as a normal DATA PPDU other than ACK and theBlock ACK is to be transmitted starting one SIFS after the end of thecorresponding PPDU.

Hereinafter, it is proposed a channel access scheme when a plurality ofSTAs operate in a Power Save (PS) mode under dense WLAN environments.

A STA operating in the PS mode transitions between an awake state and adoze state. In the awake state, the STA is fully powered. In the dozestate, the STA is not able to transmit or receive and consumes very lowpower. When operating in the PS mode, the STA listens to selected Beaconframes and sends PS-Poll frames to the AP if the TIM element in the mostrecent Beacon frame indicates an individually addressed bufferable unit(BU) is buffered for that STA. The AP transmits buffered individuallyaddressed BUs to the STA only in response to the PS-Poll frame. The STAin the doze state may enter the awake state to receive selected Beaconframes.

An operation of an HEW STA operating in a PS mode is as follows. An STAwhich has transitioned from a doze state to an awake state for frametransmission may perform a CCA process until: 1) a Network AllocationVector (NAV) of the STA is correctly set by detecting a sequence for acertain frame; or 2) a duration corresponding to ProbeDelay elapses.

However, with the use of techniques such as Beam-forming, Multi-channel,MIMO, and OFDMA, it has become more difficult to set an NAV by correctlyreceiving a Duration field in a MAC header of an MPDU. Therefore, it isproposed to perform the CCA process by the HEW STA transitioned from theDoze state to the Awake state until at least one of the followingcondition is satisfied:

1) a sequence for a certain frame is detected so that an NAV of the STAis correctly set;

2) a signal field of a Physical Layer Convergence Protocol (PLCP) headeris correctly received so that a type of a response PPDU to betransmitted after a corresponding PPDU is correctly detected and setthrough a Response Indication field;

3) a duration corresponding to ProbeDelay elapses.

If a newly changed rule is applied to the HEW STA operating in the PSmode, power consumption can be decreased since a channel access canstart when only a signal field of a PLCP header is successfully decoded.The PLCP header may also be called as a physical header.

If a certain STA correctly receives a signal field of a physical headerand thus correctly sets a type of a Response frame to be transmittedafter a corresponding PPDU through a Response Indication field, a firstinterval can be utilized to defer a channel access without having to usea second interval even if an MPDU of a corresponding PPDU cannot besuccessfully decoded and thus a Duration field value cannot be correctlyidentified. The signal field is decoded in a physical layer but the MPDUis decoded in a MAC layer. The type of the Response frame in thereceived PPDU can be identified when only decoding in the physical layeris successful. The first interval may be shorter than the secondinterval. This is to decrease power consumption by starting a channelaccess in much quicker time. The first interval may include aDistributed coordination function (DCF) Interframe Space (DIFS) and thesecond interval may include an Extended Interframe space (EIFS).

Interframe space (IFS) is a time interval between frames and is used todefer a channel access. A STA determines whether a wireless medium isbusy or idle through the use of the carrier sense (CS) function. Whenthe wireless medium is busy, the STA defers the access of the mediumduring a DIFS or an EIFS. A STA can determine that the medium is busywhen a correctly received frame is received. After DIFS expires, the STAtries to access the medium. The correctly received frame is a frame thathas successfully decoded. A STA can determine that the medium is busywhen an incorrectly received frame is received. After EIFS expires, theSTA tries to access the medium. The incorrectly received frame is aframe that has unsuccessfully decoded.

In an embodiment, an intermediately received frame is defined. Theintermediately received frame is a frame that has successfully decodedin a physical layer but has unsuccessfully decoded in a MAC layer. Thismeans that a STA can decode a signal field of the frame and can obtainthe Response Indication field to identify the type of the Responseframe. If the STA receives the intermediately received frame after theSTA transitions from a doze state to an awake state, the STA may deferthe channel access not during the EIFS but during the DIFS. Since theDIFS is shorter than the EIFS, the STA can access the medium faster.

FIG. 26 is a block diagram of an STA according to an embodiment of thepresent invention.

The STA may include a processor 21, a memory 22, and a Radio Frequency(RF) module 23.

The processor 21 implements an operation of the STA according to theembodiment of the present invention. The processor 21 may generate aPPDU according to an embodiment of the present invention and mayinstruct the RF module 23 to transmit the PPDU. The memory 22 storesinstructions for the operation of the processor 21. The storedinstructions may be executed by the processor 21 and may be implementedto perform the aforementioned operation of the STA. The RF module 23transmits and receives a radio signal.

The processor may include Application-Specific Integrated Circuits(ASICs), other chipsets, logic circuits, and/or data processors. Thememory may include Read-Only Memory (ROM), Random Access Memory (RAM),flash memory, memory cards, storage media and/or other storage devices.The RF unit may include a baseband circuit for processing a radiosignal. When the above-described embodiment is implemented in software,the above-described scheme may be implemented using a module (process orfunction) which performs the above function. The module may be stored inthe memory and executed by the processor. The memory may be disposed tothe processor internally or externally and connected to the processorusing a variety of well-known means.

In the above exemplary systems, although the methods have been describedon the basis of the flowcharts using a series of the steps or blocks,the present invention is not limited to the sequence of the steps, andsome of the steps may be performed at different sequences from theremaining steps or may be performed simultaneously with the remainingsteps. Furthermore, those skilled in the art will understand that thesteps shown in the flowcharts are not exclusive and may include othersteps or one or more steps of the flowcharts may be deleted withoutaffecting the scope of the present invention.

What is claimed is:
 1. A method for communicating in a wireless localarea network (LAN), the method comprising: transmitting, at an accesspoint (AP), a beacon frame, the beacon frame comprising at least onebasic service set identifier (BSSID) and a first color value, whereineach BSSID identifying a basic service set (BSS) managed by the AP, andthe first color value represented by a value number of bits, the valuenumber less than 48; transmitting, at the AP, a physical layer protocoldata unit (PPDU) over a transmission bandwidth, the PPDU comprising afirst signal field and a second signal field, wherein the first signalfield comprising bandwidth information and a second color value, thebandwidth information indicating the transmission bandwidth, and thesecond signal field comprising user-specific information comprisingallocation for orthogonal frequency division multiple access (OFDMA)transmission; wherein the first signal field is phase-rotated over atleast one first subcarrier group, each first subcarrier group havingbandwidth of 20 MHz within the transmission bandwidth, and the secondsignal is phase rotated over a plurality of second subcarrier groups,each second subcarrier group having bandwidth that is less than 20 MHzwithin the transmission bandwidth, and wherein the second color valuematches at least a portion of the first color value.
 2. The method ofclaim 1 wherein the PPDU is a multi-user physical layer protocol dataunit (MU-PPDU).
 3. The method of claim 1 wherein the beacon framecomprises a plurality of BSSIDs, each BSSID identifying a BSS, each BSSassociated with the first color value.
 4. The method of claim 1 whereinthe second color value includes a portion of the first color value.
 5. Adevice configured for communicating in a wireless local area network(LAN), the device comprising: a radio frequency module configured totransmit and receive radio signals; a processor operatively coupled withthe radio frequency module; and memory disposed to said processor, saidmemory including instructions, when executed by said processor cause thedevice to: transmit a beacon frame, the beacon frame comprising at leastone basic service set identifier (BSSID) and a first color value,wherein each BSSID identifying a basic service set (BSS) managed by theAP, and the first color value represented by a value number of bits, thevalue number less than 48; transmit a physical layer protocol data unit(PPDU) over a transmission bandwidth, the PPDU comprising a first signalfield and a second signal field, wherein the first signal fieldcomprising bandwidth information and a second color value, the bandwidthinformation indicating the transmission bandwidth, and the second signalfield comprising user-specific information comprising allocation fororthogonal frequency division multiple access (OFDMA) transmission; andwherein the first signal field is phase-rotated over at least one firstsubcarrier group, each first subcarrier group having bandwidth of 20 MHzwithin the transmission bandwidth, and the second signal is phaserotated over a plurality of second subcarrier groups, each secondsubcarrier group having bandwidth that is less than 20 MHz within thetransmission bandwidth, and wherein the second color value matches atleast a portion of the first color value.
 6. The device of claim 5wherein the PPDU is a multi-user physical layer protocol data unit(MU-PPDU).
 7. The device of claim 5 wherein the beacon frame comprises aplurality of BSSIDs, each BSSID identifying a BSS, each BSS associatedwith the first color value.
 8. The device of claim 5 wherein the secondcolor value includes a portion of the first color value.